RFT: break out IPv4 fragment reassembly locking into per-bucket locks

[Adrian Chadd]

Hi,

I've created a review for this:

https://reviews.freebsd.org/D2095

It does a couple things:

* The IPv4 reassembly locking is now per-bucket, rather than global
* If space needs to be made, it's made /after/ the reassembly queue
manipulation is done.  This way it's done without the queue lock held,
rather than trying to grab a lock for bucket X whilst doing work in
bucket Y.

This dramatically reduces the lock contention when doing IPv4 fragment
reassembly.

I'd prefer this to be done along RSS bucket lines so there's /no/ lock
contention during RSS based IPv4 fragment reassembly, but this is a
good first step and applies even if RSS isn't being done.

I'd appreciate any testing/reviews people may have.

Thanks,


-adrian