CIFS: Make wait_for_free_request killable

to let us kill the proccess if it hangs waiting for a credit when
the session is down and echo is disabled.

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <sfrench@us.ibm.com>