libceph: avoid a use-after-free during map check

Sending map check after complete_request() was called is not only
useless, but can lead to a use-after-free as req->r_kref decrement in
__complete_request() races with map check code.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>