Linux: Detect undefined symbols in kernel modules

Undefined symbols in the kernel modules are not currently detected
at build time.  As a result, buildbot may indicate success while
the resulting kernel modules are unusable.

In the kernel build process, modpost warns about missing symbols
but does not return an error in the case of external modules.
Detect these warnings and cause the libafs build to fail.

Reviewed-on: http://gerrit.openafs.org/8981
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit dbdee60a83d72d38a04a0eeb2a5324cf497e57e3)

Change-Id: I1986dfd3c195be13b4014014b42eb745f9daf1f8
Reviewed-on: http://gerrit.openafs.org/9364
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>