rbd: only get snap context for write requests

Right now we get the snapshot context for an rbd image (under
protection of the header semaphore) for every request processed.

There's no need to get the snap context if we're doing a read,
so avoid doing so in that case.

Note that we no longer need to hold the header semaphore to
check the rbd_dev's existence flag.

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