xfs: add a short cut to xfs_dialloc for the non-NULL agbp case

In this case we already have selected an AG and know it has free space
beause the buffer lock never got released.  Jump directly into xfs_dialloc_ag
and short cut the AG selection loop.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>