ObjectCacher: remove all buffers from a non-existent object

Once we're sure an object doesn't exist, we retry all the waiters in
order, and they return -ENOENT immediately. If there were a bunch of
BufferHeads waiting for data (rx state), they would be left behind
while the reads that triggered them were complete from the cache
user's perspective. These extra rx BufferHeads would pin the object in
the lru, so they wouldn't be removed by release_set(). This meant that
the assert during shutdown of the cache would be triggered.

To fix this, remove any BufferHeads in this state immediately when we
find out the object doesn't exist. Use the same condition as readx for
determining whether this is safe - if we got -ENOENT and all
BufferHeads for the object are clean or rx.

Fixes: #3664
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>