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.

Change-Id: I0ec45aac961dcefa0856a15ee218085626a357c7
Reviewed-on: https://gerrit.openafs.org/12663
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>