OSD: wake_pg_waiters after dropping pg lock

Otherwise, we dispatch_session_waiting while still holding the pg lock,
which is obviously wrong.  Unfortunately, this places an additional
burden on any user of _create_lock_pg, but I think it's unavoidable
since that method must atomically add the pg to the map and lock it.

Fixes: #8961
Introduced in:
  25466839589813047c975e44e67e14f34e32139e
  ecda2fef8ce982df3581a3b47ba74ae581d82479
Signed-off-by: Samuel Just <sam.just@inktank.com>