rbd: rename __rbd_add_snap_dev()

Rename __rbd_add_snap_dev() to be rbd_snap_create().  We no longer
have devices for non-mapped snapshots, and we're not actually
"adding" it to the list in this function, just creating it.

Rename rbd_remove_snap_dev() to be rbd_snap_destroy() for reasons
similar to the above.  Stop having this function delete the snapshot
from its list (to be symmetrical with its create counterpart) and do
that in the caller instead.

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