ocfs2: Use buckets in ocfs2_xattr_create_index_block().

Use the ocfs2_xattr_bucket abstraction in
ocfs2_xattr_create_index_block() and its helpers.  We get more efficient
reads, a lot less buffer_head munging, and nicer code to boot.  While
we're at it, ocfs2_xattr_update_xattr_search() becomes void.

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