nfsd4: fix bad pointer on failure to find delegation

In case of a nonempty list, the return on error here is obviously bogus;
it ends up being a pointer to the list head instead of to any valid
delegation on the list.

In particular, if nfsd4_delegreturn() hits this case, and you're quite unlucky,
then renew_client may oops, and it may take an embarassingly long time to
figure out why. Facepalm.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000090
IP: [<ffffffff81292965>] nfsd4_delegreturn+0x125/0x200
...

Cc: [e-mail]
Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfs4state.c

···
 static struct nfs4_delegation *
 find_delegation_file(struct nfs4_file *fp, stateid_t *stid)
 {
-	struct nfs4_delegation *dp = NULL;
+	struct nfs4_delegation *dp;
 
 	spin_lock(&recall_lock);
-	list_for_each_entry(dp, &fp->fi_delegations, dl_perfile) {
-		if (dp->dl_stateid.si_stateownerid == stid->si_stateownerid)
-			break;
-	}
+	list_for_each_entry(dp, &fp->fi_delegations, dl_perfile)
+		if (dp->dl_stateid.si_stateownerid == stid->si_stateownerid) {
+			spin_unlock(&recall_lock);
+			return dp;
+		}
 	spin_unlock(&recall_lock);
-	return dp;
+	return NULL;
 }
 
 int share_access_to_flags(u32 share_access)