ext4: fix bounds check in check_xattrs() to prevent out-of-bounds access

The bounds check for the next xattr entry in check_xattrs() uses
(void *)next >= end, which allows next to point within sizeof(u32)
bytes of end. On the next loop iteration, IS_LAST_ENTRY() reads 4
bytes via *(__u32 *)(entry), which can overrun the valid xattr region.

For example, if next lands at end - 1, the check passes since
next < end, but IS_LAST_ENTRY() reads 4 bytes starting at end - 1,
accessing 3 bytes beyond the valid region.

Fix this by changing the check to (void *)next + sizeof(u32) > end,
ensuring there is always enough space for the IS_LAST_ENTRY() read
on the subsequent iteration.

Fixes: 3478c83cf26b ("ext4: improve xattr consistency checking and error reporting")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20260224231429.31361-1-kartikey406@gmail.com/T/ [v1]
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Link: https://patch.msgid.link/20260328150038.349497-1-kartikey406@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>