Don't check compat.linux.emul_path before loading linux(4)

Fix e40787f900f to make libexec/rc/rc.d/linux retrieve the sysctl
after loading the kernel module which provides it, not before.

Reported By: jkim
Sponsored By: The FreeBSD Foundation

(cherry picked from commit 07cac176fba947381c8111b8e02e8067e7fa542a)