yaffs: add strict check when call yaffs_internal_read_super

When kernel booting, mount_block_root will be called to judge
the filesystem type of root device. Then .mount function in file_system_type
structure will do the check operation. But yaffs filesystem has a
relaxed examination because as a filesystem for NAND Flash, it doesn't
examinate whether the root device is the MTD NAND device. This results
that yaffs filesystem will do mount operation even though the root device
is a MMC card with a btrfs filesystem, and will crash kernel after
mounting failed. The crash log is as below:

md: Waiting for all devices to be available before autodetect
md: If you don't use raid, use raid=noautodetect
md: Autodetecting RAID arrays.
md: autorun ...
md: ... autorun DONE.
yaffs: dev is 187695107 name is "mmcblk0p3" rw
yaffs: passed flags ""
yaffs: dev is 187695107 name is "mmcblk0p3" rw
yaffs: passed flags ""
------------[ cut here ]------------
kernel BUG at fs/yaffs2/yaffs_getblockinfo.h:30!
Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
Modules linked in:
CPU: 3 PID: 1 Comm: swapper/0 Not tainted 5.2.24-yocto-standard+ #250
Hardware name: ZynqMP ZCU102 Rev1.0 (DT)
pstate: 80000005 (Nzcv daif -PAN -UAO)
pc : yaffs_rd_chunk_tags_nand+0xf0/0x110
lr : yaffs_rd_chunk_tags_nand+0x108/0x110
sp : ffffff801003b770
x29: ffffff801003b770 x28: ffffffc876fe8000
x27: 00000000000c0000 x26: 0000000000000000
x25: 00000000ffffffe1 x24: 0000000000010000
x23: 0000000000000000 x22: ffffff8011228000
x21: 000000000000005f x20: ffffff801003b890
x19: ffffffc876fe8000 x18: ffffffffffffffff
x17: 0000000000000000 x16: 0000000000000000
x15: ffffff80112285c8 x14: ffffff801137d228
x13: ffffff801137ce74 x12: ffffff8011246000
x11: 0000000000000000 x10: ffffff801137c000
x9 : 0000000000000000 x8 : 0000000000000007
x7 : 000000000000015c x6 : ffffff801137c490
x5 : 0000000000000000 x4 : 0000000000000000
x3 : 00000000ffffffff x2 : 50c80792e0663400
x1 : 0000000000000000 x0 : 0000000000000037
Call trace:
 yaffs_rd_chunk_tags_nand+0xf0/0x110
 yaffs_summary_read+0x10c/0x2e0
 yaffs2_scan_backwards+0x28c/0xf58
 yaffs_guts_initialise+0x71c/0x7a0
 yaffs_internal_read_super.isra.20+0x4ec/0x838
 yaffs2_internal_read_super_mtd+0x2c/0x48
 mount_bdev+0x1a4/0x1e0
 yaffs2_mount+0x44/0x58
 legacy_get_tree+0x34/0x60
 vfs_get_tree+0x34/0x120
 do_mount+0x708/0x980
 ksys_mount+0x9c/0x110
 mount_block_root+0x128/0x29c
 mount_root+0x148/0x17c
 prepare_namespace+0x178/0x1c0
 kernel_init_freeable+0x370/0x390
 kernel_init+0x18/0x110
 ret_from_fork+0x10/0x1c
Code: d65f03c0 f00069c0 b9440400 37f00060 (d4210000)
---[ end trace 68aa0995bdf59f76 ]---
BUG: sleeping function called from invalid context at ./include/linux/percpu-rwsem.h:34
in_atomic(): 1, irqs_disabled(): 128, pid: 1, name: swapper/0
Preemption disabled at:
[<ffffff80100a4598>] debug_exception_enter+0x30/0x40
CPU: 3 PID: 1 Comm: swapper/0 Tainted: G      D           5.2.24-yocto-standard+ #250
Hardware name: ZynqMP ZCU102 Rev1.0 (DT)
Call trace:
 dump_backtrace+0x0/0x148
 show_stack+0x24/0x30
 dump_stack+0x98/0xbc
 ___might_sleep+0x130/0x188
 __might_sleep+0x58/0x90
 exit_signals+0x44/0x258
 do_exit+0xb4/0xa38
 die+0x1bc/0x1e0
 bug_handler+0x48/0x98
 call_break_hook+0x7c/0xa8
 brk_handler+0x28/0x68
 do_debug_exception+0xc4/0x188
 el1_dbg+0x18/0x8c
 yaffs_rd_chunk_tags_nand+0xf0/0x110
 yaffs_summary_read+0x10c/0x2e0
 yaffs2_scan_backwards+0x28c/0xf58
 yaffs_guts_initialise+0x71c/0x7a0
 yaffs_internal_read_super.isra.20+0x4ec/0x838
 yaffs2_internal_read_super_mtd+0x2c/0x48
 mount_bdev+0x1a4/0x1e0
 yaffs2_mount+0x44/0x58
 legacy_get_tree+0x34/0x60
 vfs_get_tree+0x34/0x120
 do_mount+0x708/0x980
 ksys_mount+0x9c/0x110
 mount_block_root+0x128/0x29c
 mount_root+0x148/0x17c
 prepare_namespace+0x178/0x1c0
 kernel_init_freeable+0x370/0x390
 kernel_init+0x18/0x110
 ret_from_fork+0x10/0x1c
note: swapper/0[1] exited with preempt_count 1
Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
SMP: stopping secondary CPUs
Kernel Offset: disabled
CPU features: 0x0002,20002004
Memory Limit: none
---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b ]---

Use yaffs_get_mtd_device to add strict check.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>