Remove ability to write to struct if_data residing in struct ifnet
via net.link.generic.IFMIB_IFDATA.*.IFDATA_GENERAL sysctl. Reasons
for removal are:
- No code in tree uses this possibility.
- The documentation ifmib(4) doesn't say that such possibility
  exist. The example provided in manual page only reads data.
- On many interfaces the feature simply doesn't work, since they
  do accounting in hardware, and overwrite if_data on tick.

Sponsored by:	Nginx, Inc.
Use macros instead of referencing struct if_data that resides in ifnet.

Sponsored by:	Nginx, Inc.