xfs: merge xfs_buf_zero and xfs_buf_iomove

xfs_buf_zero is the only caller of xfs_buf_iomove.  Remove support
for copying from or to the buffer in xfs_buf_iomove and merge the
two functions.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>