bozo: Ignore ListKeys ka_KeyCheckSum return value

With commit c04de52da4e89e15b211b4a19a3d9bc4d612b209, an error in
ka_KeyCheckSum here makes the entire BOZO_ListKeys RPC to fail. This
caused two changes:

 - That commit makes the RPC fail in situations where it did not
   before. But even if we cannot calculate the checksum, we can still
   return other information about the key, so this is undesirable.

 - It masks the previous 'code' value, returned from stat(). The
   return code of stat() is now effectively ignored, except for the
   purposes of setting st_mtime, whereas previously a failure caused
   the RPC to fail. This is a behavior change.

So, effectively revert c04de52da4e89e15b211b4a19a3d9bc4d612b209.
Explicitly cast the return value of ka_KeyCheckSum to void, to make it
clear that we are intentionally ignoring the return value, so
hopefully this will not be flagged as a warning by code analysis tools
such as coverity.

Change-Id: Iac745d7c88ed7c2d97660e6949caa63580eef6e2
Reviewed-on: http://gerrit.openafs.org/11194
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: D Brashear <shadow@your-file-system.com>