osd: fix FileStore::_do_transaction assert bug

When process OP_COLL_ADD the assert should be applied to op2 not op.

Change-Id: I3ff7463a81583225be1288befdcd275148a9fbfe
Signed-off-by: Dong Yuan <yuandong1222@gmail.com>
Merge pull request #3232 from yuandong1222/wip-fast-txn

osd: fix FileStore::_do_transaction assert bug