PG: don't query unfound on empty pgs

When the replica responds, it responds with a notify
rather than a log, which the primary then ignores since
it is already in the peer_info map.  Rather than fix that
we'll simply not send queries to peers we already know to
have no unfound objects.

Fixes: #6910
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>
(cherry picked from commit 838b6c8387087543ce50837277f7f6b52ae87d00)
Merge pull request #1539 from ceph/wip-6910-dumpling

PG: don't query unfound on empty pgs
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>
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>