xfs: clean up the xfs_alloc_compute_aligned calling convention

Pass a xfs_alloc_arg structure to xfs_alloc_compute_aligned and derive
the alignment and minlen paramters from it.  This cleans up the existing
callers, and we'll need even more information from the xfs_alloc_arg
in subsequent patches.  Based on a patch from Dave Chinner.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>