bpf: Make dead_bpf_if const

The dead_bpf_if is not subjected to be written. Make it const so that
on destructive writing to it the kernel will panic instead of silent
memory corruption.

No functional change intended.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42189

(cherry picked from commit 7a974a649848e1222a49d0d49726d06bd5c1dbd9)