ext4: add reserved GDT blocks check

ext4: add reserved GDT blocks check

We capture a NULL pointer issue when resizing a corrupt ext4 image which
is freshly clear resize_inode feature (not run e2fsck). It could be
simply reproduced by following steps. The problem is because of the
resize_inode feature was cleared, and it will convert the filesystem to
meta_bg mode in ext4_resize_fs(), but the es->s_reserved_gdt_blocks was
not reduced to zero, so could we mistakenly call reserve_backup_gdb()
and passing an uninitialized resize_inode to it when adding new group
descriptors.

 mkfs.ext4 /dev/sda 3G
 tune2fs -O ^resize_inode /dev/sda #forget to run requested e2fsck
 mount /dev/sda /mnt
 resize2fs /dev/sda 8G

 ========
 BUG: kernel NULL pointer dereference, address: 0000000000000028
 CPU: 19 PID: 3243 Comm: resize2fs Not tainted 5.18.0-rc7-00001-gfde086c5ebfd #748
 ...
 RIP: 0010:ext4_flex_group_add+0xe08/0x2570
 ...
 Call Trace:
  <TASK>
  ext4_resize_fs+0xbec/0x1660
  __ext4_ioctl+0x1749/0x24e0
  ext4_ioctl+0x12/0x20
  __x64_sys_ioctl+0xa6/0x110
  do_syscall_64+0x3b/0x90
  entry_SYSCALL_64_after_hwframe+0x44/0xae
 RIP: 0033:0x7f2dd739617b
 ========

The fix is simple, add a check in ext4_resize_begin() to make sure that
the es->s_reserved_gdt_blocks is zero when the resize_inode feature is
disabled.

Cc: stable@kernel.org
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Ritesh Harjani <ritesh.list@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220601092717.763694-1-yi.zhang@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>