xfs: fix forkoff miscalculation related to XFS_LITINO(mp)

Currently, commit e9e2eae8 dropped a (int) decoration from
XFS_LITINO(mp), and since sizeof() expression is also involved,
the result of XFS_LITINO(mp) is simply as the size_t type
(commonly unsigned long).

Considering the expression in xfs_attr_shortform_bytesfit():
  offset = (XFS_LITINO(mp) - bytes) >> 3;
let "bytes" be (int)340, and
    "XFS_LITINO(mp)" be (unsigned long)336.

on 64-bit platform, the expression is
  offset = ((unsigned long)336 - (int)340) >> 3 =
           (int)(0xfffffffffffffffcUL >> 3) = -1

but on 32-bit platform, the expression is
  offset = ((unsigned long)336 - (int)340) >> 3 =
           (int)(0xfffffffcUL >> 3) = 0x1fffffff
instead.

so offset becomes a large positive number on 32-bit platform, and
cause xfs_attr_shortform_bytesfit() returns maxforkoff rather than 0.

Therefore, one result is
  "ASSERT(new_size <= XFS_IFORK_SIZE(ip, whichfork));"

assertion failure in xfs_idata_realloc(), which was also the root
cause of the original bugreport from Dennis, see:
   https://bugzilla.redhat.com/show_bug.cgi?id=1894177



And it can also be manually triggered with the following commands:
  $ touch a;
  $ setfattr -n user.0 -v "`seq 0 80`" a;
  $ setfattr -n user.1 -v "`seq 0 80`" a

on 32-bit platform.

Fix the case in xfs_attr_shortform_bytesfit() by bailing out
"XFS_LITINO(mp) < bytes" in advance suggested by Eric and a misleading
comment together with this bugfix suggested by Darrick. It seems the
other users of XFS_LITINO(mp) are not impacted.

Fixes: e9e2eae8 ("xfs: only check the superblock version for dinode size calculation")
Cc: <stable@vger.kernel.org> # 5.7+
Reported-and-tested-by: Dennis Gilmore <dgilmore@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>