xfs: move allocation accounting to xfs_alloc_vextent_set_fsbno()

Move it from xfs_alloc_ag_vextent() so we can get rid of that layer.
Rename xfs_alloc_vextent_set_fsbno() to xfs_alloc_vextent_finish()
to indicate that it's function is finishing off the allocation that
we've run now that it contains much more functionality.

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