xfs: consolidate realtime allocation arguments

Consolidate the arguments passed around the rt allocator into a
struct xfs_rtalloc_arg similar to how the btree allocator arguments
are consolidated in a struct xfs_alloc_arg....

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

Author: dgchinner