cifs: small variable name cleanup

server and ses->server are the same, but it's a little bit ugly that we
lock &ses->server->srv_mutex and unlock &server->srv_mutex.  It causes
a false positive in Smatch about inconsistent locking.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>
Signed-off-by: Steve French <smfrench@gmail.com>