osdc/Objecter: For func op_cancel_writes it can directly call op_cancel.

osdc/Objecter: For func op_cancel_writes it can directly call op_cancel.

Becasue we get write-lock of rwlock, so it is safe to call op_cancel
rather than _op_canchel(homeless_session for this case don't met).

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
(cherry picked from commit e4ce619fe17a7a9dfc18e6af0b84928aa2d88c00)

Conflicts:
      src/osdc/Objecter.cc: the last if (found) was incorrectly
      implemented as if (to_cancel.size()) after hammer. The
      conflict is because the patch tries to revert an incorrect
      if (to_cancel.size()) which is not in hammer.