linux-testing: Fix arg list too long in modinst

With the default kernel and thus with the build I have tested in
74ec94bf

, we get an error during
modules_install:

make[2]: execvp: /nix/store/.../bin/bash: Argument list too long

I haven't noticed this build until I actually tried booting using this
kernel because make didn't fail here.

The reason this happens within Nix and probably didn't yet surface in
other distros is that programs only have a limited amount of memory
available for storing the environment and the arguments.

Environment variables however are quite common on Nix and thus we
stumble on problems like this way earlier - in this case Linux 4.8 - but
I have noticed this in 4.7-next as well already.

The fix is far from perfect and suffers performance overhead because we
now run grep for every *.mod file instead of passing all *.mod files
into one single invocation of grep.

But comparing the performance overhead (around 1s on my machine) with
the overall build time of the kernel I think the overhead really is
neglicible.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>