NFSv4.1: Fix wraparound issues in pnfs_seqid_is_newer()

Subtraction of signed integers does not have well defined wraparound
semantics in the C99 standard. In order to be wraparound-safe, we
have to use unsigned subtraction, and then cast the result.
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/pnfs.c

@@ -662,7 +662,7 @@ pnfs_destroy_all_layouts(struct nfs_client *clp)
  */
 static bool pnfs_seqid_is_newer(u32 s1, u32 s2)
 {
-	return (s32)s1 - (s32)s2 > 0;
+	return (s32)(s1 - s2) > 0;
 }
 
 /* update lo->plh_stateid with new if is more recent */