librbd: fix rollback size

The duplicate calls to get_image_size() and get_snap_size() replaced
by 5806226cf0743bb44eaf7bc815897c6846d43233 uncovered this. The first
call was using the currently set snap_id instead of the snapshot being
rolled back to.

Fixes: #4272
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>