Fix read performance regression in ObjectCacher

The regression was introduced in commit
4fc9fffc494abedac0a9b1ce44706343f18466f1.  The problem is that the cache
thinks it's full (when it's not), so it defers the read.  This change
frees up cache space if necessary and only defers the read if enough
space cannot be freed.

Fixes: 9513
Signed-off-by: Adam Crume <adamcrume@gmail.com>
Merge pull request #2672 from ceph/wip-9513

Fix read performance regression in ObjectCacher

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>