xfs: use XFS_BUF_DADDR_NULL for daddrs in getfsmap code

Use XFS_BUF_DADDR_NULL (instead of a magic sentinel value) to mean "this
field is null" like the rest of xfs.

Cc: wozizhi@huawei.com
Fixes: e89c0413 ("xfs: implement the GETFSMAP ioctl")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>