os/bluestore: buffer cache_private field, returned by discard

os/bluestore: buffer cache_private field, returned by discard

2Q (and likly MQ?) need to know when we re-add a block that was previously
evicted.  We track these as STATE_EMPTY buffers, but we throw them out
with _discard before we add the (re)read data into the BufferSpace. Add
a Buffer field called cache_private that is returned by discard (actually,
a max over everything that is discarded) that a cache implementation can
use to discover it has re-read a recently evicted item.

The Cache implemention needs to set cache_private values such that the MAX
over them is what it actually wants...

Signed-off-by: Sage Weil <sage@redhat.com>