nfsd: allow fh_want_write to be called twice

A fuzzer recently triggered lockdep warnings about potential sb_writers
deadlocks caused by fh_want_write().

Looks like we aren't careful to pair each fh_want_write() with an
fh_drop_write().

It's not normally a problem since fh_put() will call fh_drop_write() for
us.  And was OK for NFSv3 where we'd do one operation that might call
fh_want_write(), and then put the filehandle.

But an NFSv4 protocol fuzzer can do weird things like call unlink twice
in a compound, and then we get into trouble.

I'm a little worried about this approach of just leaving everything to
fh_put().  But I think there are probably a lot of
fh_want_write()/fh_drop_write() imbalances so for now I think we need it
to be more forgiving.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/vfs.h

@@ -120,8 +120,11 @@ void		nfsd_put_raparams(struct file *file, struct raparms *ra);
 
 static inline int fh_want_write(struct svc_fh *fh)
 {
-	int ret = mnt_want_write(fh->fh_export->ex_path.mnt);
+	int ret;
 
+	if (fh->fh_want_write)
+		return 0;
+	ret = mnt_want_write(fh->fh_export->ex_path.mnt);
 	if (!ret)
 		fh->fh_want_write = true;
 	return ret;