libceph: MOSDOpReply v7 encoding

Empty request_redirect_t (struct ceph_request_redirect in the kernel
client) is now encoded with a bool.  NEW_OSDOPREPLY_ENCODING feature
bit overlaps with already supported CRUSH_TUNABLES5.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>