libceph: make cancel_generic_request() static

Fixes the following sparse warning:

net/ceph/mon_client.c:577:6: warning:
 symbol 'cancel_generic_request' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>