xfs: move and rename xfs_btree_read_bufl

Despite its name, xfs_btree_read_bufl doesn't contain any btree-related
functionaliy and isn't used by the btree code.  Move it to xfs_bmap.c,
hard code the refval and ops arguments and rename it to
xfs_bmap_read_buf.

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>