The feature gate enables mounting with -o context=XYZ mount option for all
volume types, not only ReadWriteOncePod.
All SELinux label tracking & error reporting infrastructure is already in
place from SELinuxMountReadWriteOncePod feature gate. This is just a
trivial extension to all access modes.
github.com/opencontainers/selinux/go-selinux needs OS that supports SELinux
and SELinux enabled in it to return useful data, therefore add an interface
in front of it, so we can mock its behavior in unit tests.
