ext4_utils: xattr_assert_sane false positives

ext4_utils: xattr_assert_sane false positives

xattr_free_space() does not match kernel, it subtracts
sizeof(__u32) as a buffer zone to permit IS_LAST_ENTRY
to function. As documented in .../linux/fs/ext4/xattr.c:

+------------------+
| header           |
| entry 1          | |
| entry 2          | | growing downwards
| entry 3          | v
| four null bytes  | <<<< HERE is why sizeof(__u32)
| . . .            |
| value 1          | ^
| value 3          | | growing upwards
| value 2          | |
+------------------+

Change-Id: Iee6e4e90664a3c2c5056006870b0f2f22b236bad