ocfs2: Use buckets in ocfs2_xattr_set_entry_in_bucket().

The ocfs2_xattr_set_entry_in_bucket() function is already working on an
ocfs2_xattr_bucket structure, so let's use the bucket API.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>