xfs: remove xfs_btree_reada_bufs

xfs_btree_reada_bufl just wraps xfs_btree_readahead and a agblock
to daddr conversion.  Just open code it's three callsites in the
two callers (One of which isn't even btree related).

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