xfs: refactor xfs_ialloc_ag_select

Loop over the in-core perag structures and prefer using pagi_freecount over
going out to the AGI buffer where possible.

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>