LINUX: Avoid check for key_type.match existence

Commit b5de4a9f removed our key_type 'match' function for kernels that
do not have such a 'match' function pointer. However, this added a
configure test where we are supposed to fail for the "new" behavior,
which is discouraged.

This causes an actual problem, because this test will fail on at least
RHEL5, due to arguably unrelated reasons (the header file for the
relevant struct is in key.h instead of key-type.h). And so, in that
situation we avoid defining a 'match' function callback, meaning our
'match' function callback is NULL, which causes a panic when we try to
actually look up keys for a PAG.

To fix this, transform the 'match' config test into one where we
succeed for the "new" behavior. We do this by testing for the
existence of the new functionality that replaced the old 'match'
function, which is the match_preparse function (specifically, the
'cmp' field in the structure accepted by match_preparse). This should
cause unrelated compilation errors to cause us to revert to the "old"
behavior instead of the "new" behavior. At worst, this should cause
build issues if we get the config test wrong (since we will try to use
the 'match' function definition that does not exist), instead of
panicing at runtime.

Note that while we test for key_type.match_preparse, we don't actually
use that function, since our 'match' functionality is the same as the
default behavior (according to b5de4a9f). So, we can avoid defining
any such function for newer kernels.

Thanks to Stephan Wiesand for bisecting this issue.

Reviewed-on: http://gerrit.openafs.org/11589
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit a9a3cb2efff7e6c020be4687b004d157bc070ac6)

Change-Id: I59f40258c5ea35a59681f436095922d111e344f6
Reviewed-on: http://gerrit.openafs.org/11595
Tested-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Anders Kaseorg <andersk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>