rbd: assume single op in a request

We now know that every of rbd_req_sync_op() passes an array of
exactly one operation, as evidenced by all callers passing 1 as its
num_op argument.  So get rid of that argument, assuming a single op.

Similarly, we now know that all callers of rbd_do_request() pass 1
as the num_op value, so that parameter can be eliminated as well.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>