osd/osd_types: replace bool lost with a flags field

This is more generic.  We could also fold uses_tmap flag into here,
but the encoding change for that is non-trivial.

Signed-off-by: Sage Weil <sage@inktank.com>
osd/osd_types: add WHITEOUT flag to object_info_t

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: update all find_object_context() users to handle whiteouts

In each case, we treat the whiteout as if we got an ENOENT.

We do not change the semantics of bool exists to avoid breaking lots of
potentially fragile code.  We are only interested in changing the
user-visible behavior of the object, not the way it is internally stored
or managed.

This will likely be refined as we grow acutal users for whiteoutes in the
pool caching code.

Signed-off-by: Sage Weil <sage@inktank.com>
osd/osd_types: convert object_info_t::uses_tmap to a flag

Treat the second encoded bool as bits 9-16 of a (now) 16-bit flags field,
and use bit 9 (what used to be set by the use_tmap bool) as FLAG_USES_TMAP.

No encoding compatibility change.

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