librbd: discard cache state if compare-and-write writes to disk

librbd: discard cache state if compare-and-write writes to disk

Otherwise this is obviously broken with rbd_cache_policy = writeback or
rbd_cache_policy = writethrough as the write is done on the OSD, beneath
ObjectCacher.  This went unnoticed because the original (and so far the
only known) compare-and-write user, tcmu-runner rbd driver, disables the
cache altogether.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>