pf: Virtualise pf_frag_mtx

The pf_frag_mtx mutex protects the fragments queue. The fragments queue
is virtualised already (i.e. per-vnet) so it makes no sense to block
jail A from accessing its fragments queue while jail B is accessing its
own fragments queue.

Virtualise the lock for improved concurrency.

Differential Revision: https://reviews.freebsd.org/D24504