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.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270868 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Use macros instead of referencing struct if_data that resides in ifnet.

Sponsored by:	Nginx, Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270869 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f