reiserfs: add check for root_inode in reiserfs_fill_super

Our syzcaller report a NULL pointer dereference:

BUG: kernel NULL pointer dereference, address: 0000000000000000
PGD 116e95067 P4D 116e95067 PUD 1080b5067 PMD 0
Oops: 0010 [#1] SMP KASAN
CPU: 7 PID: 592 Comm: a.out Not tainted 5.13.0-next-20210629-dirty #67
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-p4
RIP: 0010:0x0
Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
RSP: 0018:ffff888114e779b8 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 1ffff110229cef39 RCX: ffffffffaa67e1aa
RDX: 0000000000000000 RSI: ffff88810a58ee00 RDI: ffff8881233180b0
RBP: ffffffffac38e9c0 R08: ffffffffaa67e17e R09: 0000000000000001
R10: ffffffffb91c5557 R11: fffffbfff7238aaa R12: ffff88810a58ee00
R13: ffff888114e77aa0 R14: 0000000000000000 R15: ffff8881233180b0
FS:  00007f946163c480(0000) GS:ffff88839f1c0000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 00000001099c1000 CR4: 00000000000006e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __lookup_slow+0x116/0x2d0
 ? page_put_link+0x120/0x120
 ? __d_lookup+0xfc/0x320
 ? d_lookup+0x49/0x90
 lookup_one_len+0x13c/0x170
 ? __lookup_slow+0x2d0/0x2d0
 ? reiserfs_schedule_old_flush+0x31/0x130
 reiserfs_lookup_privroot+0x64/0x150
 reiserfs_fill_super+0x158c/0x1b90
 ? finish_unfinished+0xb10/0xb10
 ? bprintf+0xe0/0xe0
 ? __mutex_lock_slowpath+0x30/0x30
 ? __kasan_check_write+0x20/0x30
 ? up_write+0x51/0xb0
 ? set_blocksize+0x9f/0x1f0
 mount_bdev+0x27c/0x2d0
 ? finish_unfinished+0xb10/0xb10
 ? reiserfs_kill_sb+0x120/0x120
 get_super_block+0x19/0x30
 legacy_get_tree+0x76/0xf0
 vfs_get_tree+0x49/0x160
 ? capable+0x1d/0x30
 path_mount+0xacc/0x1380
 ? putname+0x97/0xd0
 ? finish_automount+0x450/0x450
 ? kmem_cache_free+0xf8/0x5a0
 ? putname+0x97/0xd0
 do_mount+0xe2/0x110
 ? path_mount+0x1380/0x1380
 ? copy_mount_options+0x69/0x140
 __x64_sys_mount+0xf0/0x190
 do_syscall_64+0x35/0x80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

This is because 'root_inode' is initialized with wrong mode, and
it's i_op is set to 'reiserfs_special_inode_operations'. Thus add
check for 'root_inode' to fix the problem.

Link: https://lore.kernel.org/r/20210702040743.1918552-1-yukuai3@huawei.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>