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: [e-mail]
Fixes: e89c041338ed6 ("xfs: implement the GETFSMAP ioctl")
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Darrick J. Wong <[e-mail]>
Signed-off-by: Chandan Babu R <[e-mail]>

fs/xfs/xfs_fsmap.c

···
 	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;
···
 		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)