xfs: simplify xfs_rtbuf_get calling conventions

Now that xfs_rtalloc_args holds references to the last-read bitmap and
summary blocks, we don't need to pass the buffer pointer out of
xfs_rtbuf_get.

Callers no longer have to xfs_trans_brelse on their own, though they are
required to call xfs_rtbuf_cache_relse before the xfs_rtalloc_args goes
out of scope.

While we're at it, create some trivial helpers so that we don't have to
remember if "0" means "bitmap" and "1" means "summary".

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