NFSv4.1: Deal with wraparound issues when updating the layout stateid

...and add a helper function.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/pnfs.c

@@ -549,6 +549,15 @@ pnfs_destroy_all_layouts(struct nfs_client *clp)
 	}
 }
 
+/*
+ * Compare 2 layout stateid sequence ids, to see which is newer,
+ * taking into account wraparound issues.
+ */
+static bool pnfs_seqid_is_newer(u32 s1, u32 s2)
+{
+	return (s32)s1 - (s32)s2 > 0;
+}
+
 /* update lo->plh_stateid with new if is more recent */
 void
 pnfs_set_layout_stateid(struct pnfs_layout_hdr *lo, const nfs4_stateid *new,
@@ -558,7 +567,7 @@ pnfs_set_layout_stateid(struct pnfs_layout_hdr *lo, const nfs4_stateid *new,
 
 	oldseq = be32_to_cpu(lo->plh_stateid.seqid);
 	newseq = be32_to_cpu(new->seqid);
-	if (list_empty(&lo->plh_segs) || (int)(newseq - oldseq) > 0) {
+	if (list_empty(&lo->plh_segs) || pnfs_seqid_is_newer(newseq, oldseq)) {
 		nfs4_stateid_copy(&lo->plh_stateid, new);
 		if (update_barrier) {
 			u32 new_barrier = be32_to_cpu(new->seqid);