xfs: untangle xfs_dialloc

Clarify the control flow in xfs_dialloc.  Factor out a helper to go to the
next node from the current one and improve the control flow by expanding
composite if statements and using gotos.

The xfs_ialloc_next_rec helper is borrowed from Dave Chinners dynamic
allocation policy patches.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>