ocfs2: Make xattr reflink work with new local alloc reservation.

The new reservation code in local alloc has add the limitation
that the caller should handle the case that the local alloc
doesn't give use enough contiguous clusters. It make the old
xattr reflink code broken.

So this patch udpate the xattr reflink code so that it can
handle the case that local alloc give us one cluster at a time.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>