iflib: Fix some nits in the rx refill code.

[markj]

Summary

- Get rid of the ifl_vm_addrs array. It is not used by any existing
consumer, so we are just dirtying a couple of cache lines for no
reason.
- Use uma_zalloc(fl->ifl_zone) instead of m_cljget(). Otherwise
m_cljget() is doing unnecessary work to look up the correct zone, when
iflib already knows what that zone is.
- ifl_gen is only used when INVARIANTS is on, so make that more clear.
- Fix some style nits and inconsistencies.

[gallatin]

Great work. There are *so many* of these optimizations hiding in this code. I'm really glad you're looking at it.