xfs: move allocation accounting to xfs_alloc_vextent_set_fsbno()

xfs: move allocation accounting to xfs_alloc_vextent_set_fsbno()

Source kernel commit: e4d174260779ff0e2dc5de951c2e235721b02b05

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>
Signed-off-by: Carlos Maiolino <cem@kernel.org>