NFSD: Write verifier might go backwards

When vfs_iter_write() starts to fail because a file system is full,
a bunch of writes can fail at once with ENOSPC. These writes
repeatedly invoke nfsd_reset_boot_verifier() in quick succession.

Ensure that the time it grabs doesn't go backwards due to an ntp
adjustment going on at the same time.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfssvc.c

@@ -363,7 +363,7 @@ void nfsd_copy_boot_verifier(__be32 verf[2], struct nfsd_net *nn)
 
 static void nfsd_reset_boot_verifier_locked(struct nfsd_net *nn)
 {
-	ktime_get_real_ts64(&nn->nfssvc_boot);
+	ktime_get_raw_ts64(&nn->nfssvc_boot);
 }
 
 void nfsd_reset_boot_verifier(struct nfsd_net *nn)