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>

fs/xfs/xfs_fsmap.c

@@ -252,7 +252,7 @@ xfs_getfsmap_rec_before_start(
 	const struct xfs_rmap_irec	*rec,
 	xfs_daddr_t			rec_daddr)
 {
-	if (info->low_daddr != -1ULL)
+	if (info->low_daddr != XFS_BUF_DADDR_NULL)
 		return rec_daddr < info->low_daddr;
 	if (info->low.rm_blockcount)
 		return xfs_rmap_compare(rec, &info->low) < 0;
@@ -983,7 +983,7 @@ xfs_getfsmap(
 		info.dev = handlers[i].dev;
 		info.last = false;
 		info.pag = NULL;
-		info.low_daddr = -1ULL;
+		info.low_daddr = XFS_BUF_DADDR_NULL;
 		info.low.rm_blockcount = 0;
 		error = handlers[i].fn(tp, dkeys, &info);
 		if (error)