[PATCH][X/Z/A] CVE fixes for eBPF

[Seth Forshee]

The following patches fix problems with eBPF in xenial, zesty, and
artful, assigned the following CVE numbers:

 CVE-2017-16995
 CVE-2017-17862
 CVE-2017-17863
 CVE-2017-17864

These are taken from a larger set of vulnerabilities, several of which
were introduced only in 4.14. Only the first two fixes are applicable to
xenial, with some extra backporting to make the fixes compatible. The
patches for CVE-2017-17863 and CVE-2017-17864 are based on the fixes
from 4.9 stable and debian respectively as the code has diverged
sisignificatnly upstream.

Thanks,
Seth