fix some bugs.

1. fix l2tlb dead-lock bug
l2tlb won't merge requests at same addr. It will be blocked when having too many requests.
PtwFilter has a bug that will send too many requests. Add a counter to avoid that.

2. fix sfence sync at mmu
different modules in mmu may get sfence at different latency, which will lost requests or some requests have no receiver.
Sync the sfence latency manually to avoid the bug.

* mmu.filter: add counter not to send to many req to l2tlb

* mmu.filter: fix bug that forget counter signal when block issue and deq

* mmu: set sfence/csr delay to 2 cycle, must sync in mmu