cifs: Avoid umount hangs with smb2 when server is unresponsive

Do not send SMB2 Logoff command when reconnecting, the way smb1
code base works.

Also, no need to wait for a credit for an echo command when one is already
in flight.

Without these changes, umount command hangs if the server is unresponsive
e.g. hibernating.

Signed-off-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@us.ibm.com>
diff --git a/fs/cifs/transport.c b/fs/cifs/transport.c
index 6fdcb1b..800b938 100644
--- a/fs/cifs/transport.c
+++ b/fs/cifs/transport.c
@@ -410,8 +410,13 @@
 wait_for_free_request(struct TCP_Server_Info *server, const int timeout,
 		      const int optype)
 {
-	return wait_for_free_credits(server, timeout,
-				server->ops->get_credits_field(server, optype));
+	int *val;
+
+	val = server->ops->get_credits_field(server, optype);
+	/* Since an echo is already inflight, no need to wait to send another */
+	if (*val <= 0 && optype == CIFS_ECHO_OP)
+		return -EAGAIN;
+	return wait_for_free_credits(server, timeout, val);
 }
 
 static int allocate_mid(struct cifs_ses *ses, struct smb_hdr *in_buf,