xfs: move xfs_bmap_btalloc_filestreams() to xfs_filestreams.c

xfs_bmap_btalloc_filestreams() calls two filestreams functions to
select the AG to allocate from. Both those functions end up in
the same selection function that iterates all AGs multiple times.
Worst case, xfs_bmap_btalloc_filestreams() can iterate all AGs 4
times just to select the initial AG to allocate in.

Move the AG selection to fs/xfs/xfs_filestreams.c as a single
interface so that the inefficient AG interation is contained
entirely within the filestreams code. This will allow the
implementation to be simplified and made more efficient in future
patches.

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