net.link.generic.ifdata.<ifindex>.linkspecific: rework handler

This OID was added in r17352 but the write path of IFDATA_LINKSPECIFIC
seems unused as there are no in-base writers, and as far as I can tell
we had issues with this code before, see PR 219472. Drop the write path
to make the handler read-only as described in comments and man-pages.
It can be marked as MPSAFE now.

Reviewed by: bdragon, kib, melifaro, wollman
Approved by: kib (mentor)
Sponsored by: Mysterious Code Ltd.
Differential Revision: https://reviews.freebsd.org/D25348