[BUG]
Syzbot reports the following regression detected by KASAN:

  BUG: KASAN: slab-out-of-bounds in btrfs_qgroup_inherit+0x42e/0x2e20 fs/btrfs/qgroup.c:3277
  Read of size 8 at addr ffff88814628ca50 by task syz-executor318/5171

  CPU: 0 PID: 5171 Comm: syz-executor318 Not tainted 6.10.0-rc2-syzkaller-00010-g2ab79514 #0
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/02/2024
  Call Trace:
   <TASK>
   __dump_stack lib/dump_stack.c:88 [inline]
   dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114
   print_address_description mm/kasan/report.c:377 [inline]
   print_report+0x169/0x550 mm/kasan/report.c:488
   kasan_report+0x143/0x180 mm/kasan/report.c:601
   btrfs_qgroup_inherit+0x42e/0x2e20 fs/btrfs/qgroup.c:3277
   create_pending_snapshot+0x1359/0x29b0 fs/btrfs/transaction.c:1854
   create_pending_snapshots+0x195/0x1d0 fs/btrfs/transaction.c:1922
   btrfs_commit_transaction+0xf20/0x3740 fs/btrfs/transaction.c:2382
   create_snapshot+0x6a1/0x9e0 fs/btrfs/ioctl.c:875
   btrfs_mksubvol+0x58f/0x710 fs/btrfs/ioctl.c:1029
   btrfs_mksnapshot+0xb5/0xf0 fs/btrfs/ioctl.c:1075
   __btrfs_ioctl_snap_create+0x387/0x4b0 fs/btrfs/ioctl.c:1340
   btrfs_ioctl_snap_create_v2+0x1f2/0x3a0 fs/btrfs/ioctl.c:1422
   btrfs_ioctl+0x99e/0xc60
   vfs_ioctl fs/ioctl.c:51 [inline]
   __do_sys_ioctl fs/ioctl.c:907 [inline]
   __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:893
   do_syscall_x64 arch/x86/entry/common.c:52 [inline]
   do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
  RIP: 0033:0x7fcbf1992509
  RSP: 002b:00007fcbf1928218 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
  RAX: ffffffffffffffda RBX: 00007fcbf1a1f618 RCX: 00007fcbf1992509
  RDX: 0000000020000280 RSI: 0000000050009417 RDI: 0000000000000003
  RBP: 00007fcbf1a1f610 R08: 00007ffea1298e97 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000246 R12: 00007fcbf19eb660
  R13: 00000000200002b8 R14: 00007fcbf19e60c0 R15: 0030656c69662f2e
   </TASK>

And it also pinned it down to commit b5357cb2 ("btrfs: qgroup: do not
check qgroup inherit if qgroup is disabled").

[CAUSE]
That offending commit skips the whole qgroup inherit check if qgroup is
not enabled.

But that also skips the very basic checks like
num_ref_copies/num_excl_copies and the structure size checks.

Meaning if a qgroup enable/disable race is happening at the background,
and we pass a btrfs_qgroup_inherit structure when the qgroup is
disabled, the check would be completely skipped.

Then at the time of transaction commitment, qgroup is re-enabled and
btrfs_qgroup_inherit() is going to use the incorrect structure and
causing the above KASAN error.

[FIX]
Make btrfs_qgroup_check_inherit() only skip the source qgroup checks.
So that even if invalid btrfs_qgroup_inherit structure is passed in, we
can still reject invalid ones no matter if qgroup is enabled or not.

Furthermore we do already have an extra safety inside
btrfs_qgroup_inherit(), which would just ignore invalid qgroup sources,
so even if we only skip the qgroup source check we're still safe.

Reported-by: syzbot+a0d1f7e26910be4dc171@syzkaller.appspotmail.com
Fixes: b5357cb2 ("btrfs: qgroup: do not check qgroup inherit if qgroup is disabled")
Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Jeongjun Park <aha310510@gmail.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>