osdc/ObjectCacher: call read completion even when no target buffer

If we do no assemble a target bl, we still want to return a valid return
code with the number of bytes read-ahead so that the C_RetryRead completion
will see this as a finish and call the caller's provided Context.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 032d4ec53e125ad91ad27ce58da6f38dcf1da92e)
client: pin Inode during readahead

Make sure the Inode does not go away while a readahead is in progress.  In
particular:

 - read_async
   - start a readahead
   - get actual read from cache, return
 - close/release
   - call ObjectCacher::release_set() and get unclean > 0, assert

Fixes: #7867
Backport: emperor, dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit f1c7b4ef0cd064a9cb86757f17118d17913850db)