xfs: introduce xfs_buf_daddr()

Introduce a helper function xfs_buf_daddr() to extract the disk
address of the buffer from the struct xfs_buf. This will replace
direct accesses to bp->b_bn and bp->b_maps[0].bm_bn, as well as
the XFS_BUF_ADDR() macro.

This patch introduces the helper function and replaces all uses of
XFS_BUF_ADDR() as this is just a simple sed replacement.

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