linux(4): Add a helper intended for copying timespec's from the userspace.

There are many places where we copyin Linux timespec from the userspace
and then convert it to the kernel timespec. To avoid code duplication
add a tiny halper for doing this.

MFC after: 2 weeks

(cherry picked from commit 707e567a4061669b7643ddbe8a1ad9c4e6545a75)