NFSD: use read_seqbegin() rather than read_seqbegin_or_lock()

The usage of read_seqbegin_or_lock() in nfsd_copy_write_verifier()
is wrong. "seq" is always even and thus "or_lock" has no effect,
this code can never take ->writeverf_lock for writing.

I guess this is fine, nfsd_copy_write_verifier() just copies 8 bytes
and nfsd_reset_write_verifier() is supposed to be very rare operation
so we do not need the adaptive locking in this case.

Yet the code looks wrong and sub-optimal, it can use read_seqbegin()
without changing the behaviour.

[ cel: Note also that it eliminates this Sparse warning:

fs/nfsd/nfssvc.c:360:6: warning: context imbalance in 'nfsd_copy_write_verifier' -
	different lock contexts for basic block

]
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfssvc.c

@@ -359,13 +359,12 @@ static bool nfsd_needs_lockd(struct nfsd_net *nn)
  */
 void nfsd_copy_write_verifier(__be32 verf[2], struct nfsd_net *nn)
 {
-	int seq = 0;
+	unsigned int seq;
 
 	do {
-		read_seqbegin_or_lock(&nn->writeverf_lock, &seq);
+		seq = read_seqbegin(&nn->writeverf_lock);
 		memcpy(verf, nn->writeverf, sizeof(nn->writeverf));
-	} while (need_seqretry(&nn->writeverf_lock, seq));
-	done_seqretry(&nn->writeverf_lock, seq);
+	} while (read_seqretry(&nn->writeverf_lock, seq));
 }
 
 static void nfsd_reset_write_verifier_locked(struct nfsd_net *nn)