libceph: pass num_op with ops

Both ceph_osdc_alloc_request() and ceph_osdc_build_request() are
provided an array of ceph osd request operations.  Rather than just
passing the number of operations in the array, the caller is
required append an additional zeroed operation structure to signal
the end of the array.

All callers know the number of operations at the time these
functions are called, so drop the silly zero entry and supply that
number directly.  As a result, get_num_ops() is no longer needed.
This also means that ceph_osdc_alloc_request() never uses its ops
argument, so that can be dropped.

Also rbd_create_rw_ops() no longer needs to add one to reserve room
for the additional op.

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