cifs: protect GlobalOplock_Q with its own spinlock

Right now, the GlobalOplock_Q is protected by the GlobalMid_Lock. That
lock is also used for completely unrelated purposes (mostly for managing
the global mid queue). Give the list its own dedicated spinlock
(cifs_oplock_lock) and rename the list to cifs_oplock_list to
eliminate the camel-case.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>