xfs: use XFS_BUF_DADDR_NULL for daddrs in getfsmap code

xfs: use XFS_BUF_DADDR_NULL for daddrs in getfsmap code

[ Upstream commit 6b35cc8d9239569700cc7cc737c8ed40b8b9cfdb ]

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: e89c041338ed6 ("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>
Signed-off-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>