libceph: make encode_request_*() work with r_mempool requests

Messages allocated out of ceph_msgpool have a fixed front length
(pool->front_len).  Asserting that the entire front has been filled
while encoding is thus wrong.

Fixes: 8cb441c0 ("libceph: MOSDOp v8 encoding (actual spgid + full hash)")
Reported-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>