xfs: fold xfs_alloc_ag_vextent() into callers

We don't need the multiplexing xfs_alloc_ag_vextent() provided
anymore - we can just call the exact/near/size variants directly.
This allows us to remove args->type completely and stop using
args->fsbno as an input to the allocator algorithms.

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