NFSv4/pNFS: Always allow update of a zero valued layout barrier

A zero value for the layout barrier indicates that it has been cleared
(since seqid '0' is an illegal value), so we should always allow it to
be updated.

Fixes: d29b468da4 ("pNFS/NFSv4: Improve rejection of out-of-order layouts")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/pnfs.c

@@ -335,7 +335,7 @@ static bool pnfs_seqid_is_newer(u32 s1, u32 s2)
 
 static void pnfs_barrier_update(struct pnfs_layout_hdr *lo, u32 newseq)
 {
-	if (pnfs_seqid_is_newer(newseq, lo->plh_barrier))
+	if (pnfs_seqid_is_newer(newseq, lo->plh_barrier) || !lo->plh_barrier)
 		lo->plh_barrier = newseq;
 }