Check at modules_install whether depmod ignores the extra subdir

Check at modules_install whether depmod ignores the extra subdir

The external kernel modules are installed in the extra subdir in
/lib/modules/<kernelrelease>/, but depmod in some distributions
(at least in Ubuntu 12.04 LTS) ingores the subdir. Warn about it,
because that way the modules are actually not available there.

Reported by Husnu Demir and tian fang.