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