src/attr_replace_test: limit size of extended attribute value

src/attr_replace_test: limit size of extended attribute value

The maximum size for extended attribute values is 65536 (XATTR_SIZE_MAX).
Since there are filesystems that can set blksize to really big values
(CephFS for example has a default of 4M), it's easy to have this test
failing with fsetxattr returning -E2BIG.

Cc: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>