ocfs2: Move buckets up into ocfs2_add_new_xattr_bucket().

Lift the buckets from ocfs2_add_new_xattr_cluster() up into
ocfs2_add_new_xattr_bucket().  Now ocfs2_add_new_xattr_cluster()
doesn't deal with buffer_heads.  In fact, we no longer have to play
get_bh() tricks at all.

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