os/bluestore: fix SharedBlobSet deregister race

os/bluestore: fix SharedBlobSet deregister race

In commit 8c8944b2c45ca9dc5b8fd4db1590e1d24206c0b3 we fixed one narrow race
and introduced a new (probably less narrow) one:

A: put shared blob foo, nref = 0, start removing self from set
B: open_shared_blob -> lookup gets nullptr (nref==null), creates a new shared blob
  B: takes lock, sets sb_map[sbid] = newblob
A: gets lock, erases sb_map[sbid]
B: open_shared_blob -> lookup gets null, creates another new shared blob

Fix by only removing the sb_map entry for the nref=0 sb if it still points
to us.  If it doesn't, that means some new blob has shown up in its place.

Fixes: http://tracker.ceph.com/issues/24319
Signed-off-by: Sage Weil <sage@redhat.com>