linux(4): Fix futex copyrights.

As no more NetBSD code in futexes exists replace NetBSD copyrights by
standard FreeBSD 2 clause license.
Add Roman Divacky's copyrights as an author of the robust futexes.

Differential revision: https://reviews.freebsd.org/D31347
MFC after: 2 weeks

(cherry picked from commit 6e31bed646c2f67cbcfd4bb2ce38aadd15868dcc)