iflib: remove convoluted custom zeroing code

Replace a collection of aliasing violations and ifdefs with memset
(which now expands to __builtin_memset and should be quite reliably
inlined.) The old code is hard to maintain as evidenced by the most
recent change to if_pkt_info_t updating the defines, but not the zeroing
code.

Reviewed by: gallatin, erj
Effort: CHERI upstreaming
Sponsored by: Innovate UK
Fixes: 43d7ee540efe ("iflib: support for transmit side nic KTLS offload")
Differential Revision: https://reviews.freebsd.org/D54605