Enforce cache size on read requests

In-flight cache reads were not previously counted against
new cache read requests, which could result in very large
cache usage.  This effect is most noticeable when writing
small chunks to a cloned image since each write requires
a full object read from the parent.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
Merge pull request #2422 from ceph/wip-6494

Enforce cache size on read requests

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>