rbd: pass num_op with ops array

Add a num_op parameter to rbd_do_request() and rbd_req_sync_op() to
indicate the number of entries in the array.  The callers of these
functions always know how many entries are in the array, so just
pass that information down.

This is in anticipation of eliminating the extra zero-filled entry
in these ops arrays.

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