xfs: use for_each_perag_wrap in xfs_filestream_pick_ag

xfs_filestream_pick_ag() is now ready to rework to use
for_each_perag_wrap() for iterating the perags during the AG
selection scan.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>