os/bluestore: handle bnode removal race

os/bluestore: handle bnode removal race

One thread might try to drop a bnode while another does a lookup and
retakes a ref:

 1- drop bnode ref (-> 0)
 2- take bnode_set lock, do lookup
 2- take bnode ref (-> 1)
 2- drop lock
 1- take bnode_set lock
 1- remove from set
 1- drop lock

Fix this race by rechecking the ref count while we are holding the lock.

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