Linux 4.13: use designated initializers where required

Linux 4.13: use designated initializers where required

struct path is declared with the "designated_init" attribute,
and module builds now use -Werror=designated-init. Cope.

And as pointed out by Michael Meffie, struct ctl_table has
the same requirement now, so use a designated initializer
for the final element of the sysctl table too.

Reviewed-on: https://gerrit.openafs.org/12663
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit d55b41072ce873210481baa4cae5c7143011869b)

Change-Id: I9797f8b1e16ab095775c53ac03e258affaa4d303
Reviewed-on: https://gerrit.openafs.org/12664
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Anders Kaseorg <andersk@mit.edu>
Tested-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>