os/bluestore: simplify and fix SharedBlob::put()

os/bluestore: simplify and fix SharedBlob::put()

There is a narrow race possible:

A: lookup foo
A: put on foo
A:   foo --nref == 0
B: lookup foo
B: put foo
B:   foo --nref == 0
B: try_remove() succeeds, removes
A: try_remove() tries to remove foo again, probably crashes

We could fix this by flagging the object in some way to indicate it was
removed (maybe clearing parent?), but then we need to be careful about
dereferencing foo to get parent from put().

Fix this by moving to a simpler model: make lookup fail if nref == 0.
This eliminates the races around put() entirely because once nref reaches
0 it never goes up again.

Fixes: http://tracker.ceph.com/issues/24211
Signed-off-by: Sage Weil <sage@redhat.com>