GRSecurity 3.0 compat: Make ctldir use zexec_helper

GRSecurity 3.0 restricts call_usermodehelper() to binaries in /sbin and
certain system paths. The purpose of this change is to prevent a
hypothetical vulnerability where a user with the ability to write into
the kernel modifies modprobe_path from becoming a privilege escalation
vulnerability. This breaks our usage of /bin/sh to call /bin/mount and
/bin/umount, so we use /sbin/zexec_helper to /sbin to replace our use of
/bin/sh.

Signed-off-by: Richard Yao <ryao@gentoo.org>