os/bluestore: fix a bug causing unexpected Onode's unpinned state.

os/bluestore: fix a bug causing unexpected Onode's unpinned state.

There could be a race for Onodes put() and get() methods:

put()(pinned, nref=3)
  int n = --nref; (nref = 2)
  if (n == 2) {
    ..
    std::lock_guard l(ocs->lock);
    ...
    pinned = pinned && nref > 2; (= false)
    ...                                     get()
    if (r) {                                ++nref; (=3)
      n = --nref; (nref = 2)                return;
    }
    ...
    return

As a result nref = 2, pinned = false which is wrong

Signed-off-by: Igor Fedotov <ifedotov@suse.com>