iflib: Fix some nits in the rx refill code.

iflib: Fix some nits in the rx refill code.

- 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.

Reviewed by:	gallatin
Tested by:	pho
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D25490