[CIFS] pSesInfo->sesSem is used as mutex. Rename it to session_mutex and

convert it to a real mutex.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>