iflib: Make if_shared_ctx_t a pointer to const

This structure is shared among multiple instances of a driver, so we
should ensure that it doesn't somehow get treated as if there's a
separate instance per interface. This is especially important for
software-only drivers like wg.

DEVICE_REGISTER() still returns a void * and so the per-driver sctx
structures are not yet defined with the const qualifier.

Reviewed by: gallatin, erj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29102