SELinux filesystem is at /sys/fs/selinux

The SELinux filesystem has been moved to /sys/fs/selinux for quite some
time. We kept supporting /selinux for backwards compatibility, but it's
time to move forward on this.

X-Gentoo-Bug: 511718
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=511718
Signed-off-by: Sven Vermeulen <sven.vermeulen@siphos.be>