jffs2: Fix NULL pointer dereference in rp_size fs option parsing

syzkaller found the following JFFS2 splat:

  Unable to handle kernel paging request at virtual address dfffa00000000001
  Mem abort info:
    ESR = 0x96000004
    EC = 0x25: DABT (current EL), IL = 32 bits
    SET = 0, FnV = 0
    EA = 0, S1PTW = 0
  Data abort info:
    ISV = 0, ISS = 0x00000004
    CM = 0, WnR = 0
  [dfffa00000000001] address between user and kernel address ranges
  Internal error: Oops: 96000004 [#1] SMP
  Dumping ftrace buffer:
     (ftrace buffer empty)
  Modules linked in:
  CPU: 0 PID: 12745 Comm: syz-executor.5 Tainted: G S                5.9.0-rc8+ #98
  Hardware name: linux,dummy-virt (DT)
  pstate: 20400005 (nzCv daif +PAN -UAO BTYPE=--)
  pc : jffs2_parse_param+0x138/0x308 fs/jffs2/super.c:206
  lr : jffs2_parse_param+0x108/0x308 fs/jffs2/super.c:205
  sp : ffff000022a57910
  x29: ffff000022a57910 x28: 0000000000000000
  x27: ffff000057634008 x26: 000000000000d800
  x25: 000000000000d800 x24: ffff0000271a9000
  x23: ffffa0001adb5dc0 x22: ffff000023fdcf00
  x21: 1fffe0000454af2c x20: ffff000024cc9400
  x19: 0000000000000000 x18: 0000000000000000
  x17: 0000000000000000 x16: ffffa000102dbdd0
  x15: 0000000000000000 x14: ffffa000109e44bc
  x13: ffffa00010a3a26c x12: ffff80000476e0b3
  x11: 1fffe0000476e0b2 x10: ffff80000476e0b2
  x9 : ffffa00010a3ad60 x8 : ffff000023b70593
  x7 : 0000000000000003 x6 : 00000000f1f1f1f1
  x5 : ffff000023fdcf00 x4 : 0000000000000002
  x3 : ffffa00010000000 x2 : 0000000000000001
  x1 : dfffa00000000000 x0 : 0000000000000008
  Call trace:
   jffs2_parse_param+0x138/0x308 fs/jffs2/super.c:206
   vfs_parse_fs_param+0x234/0x4e8 fs/fs_context.c:117
   vfs_parse_fs_string+0xe8/0x148 fs/fs_context.c:161
   generic_parse_monolithic+0x17c/0x208 fs/fs_context.c:201
   parse_monolithic_mount_data+0x7c/0xa8 fs/fs_context.c:649
   do_new_mount fs/namespace.c:2871 [inline]
   path_mount+0x548/0x1da8 fs/namespace.c:3192
   do_mount+0x124/0x138 fs/namespace.c:3205
   __do_sys_mount fs/namespace.c:3413 [inline]
   __se_sys_mount fs/namespace.c:3390 [inline]
   __arm64_sys_mount+0x164/0x238 fs/namespace.c:3390
   __invoke_syscall arch/arm64/kernel/syscall.c:36 [inline]
   invoke_syscall arch/arm64/kernel/syscall.c:48 [inline]
   el0_svc_common.constprop.0+0x15c/0x598 arch/arm64/kernel/syscall.c:149
   do_el0_svc+0x60/0x150 arch/arm64/kernel/syscall.c:195
   el0_svc+0x34/0xb0 arch/arm64/kernel/entry-common.c:226
   el0_sync_handler+0xc8/0x5b4 arch/arm64/kernel/entry-common.c:236
   el0_sync+0x15c/0x180 arch/arm64/kernel/entry.S:663
  Code: d2d40001 f2fbffe1 91002260 d343fc02 (38e16841)
  ---[ end trace 4edf690313deda44 ]---

This is because since ec10a24f, the option parsing happens before
fill_super and so the MTD device isn't associated with the filesystem.
Defer the size check until there is a valid association.

Fixes: ec10a24f ("vfs: Convert jffs2 to use the new mount API")
Cc: <stable@vger.kernel.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Jamie Iles <jamie@nuviainc.com>
Signed-off-by: Richard Weinberger <richard@nod.at>