linux: add check for sysctl before using it
linux: add check for sysctl before using it

check that the sysctl we'd like to use is supported by the kernel
before using it.

Closes: https://github.com/containers/buildah/issues/1618

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Closes: #1619
Approved by: rhatdan

nalind/buildah-1,rhatdan/buildah,rhatdan/buildah,projectatomic/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,nalind/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,nalind/buildah,nalind/buildah-1,rhatdan/buildah