Linux 3.17: No more typedef for ctl_table

The typedef has been removed so we need to use the structure
directly.

Note that the API for register_sysctl_table has also changed
with 3.17, but it reverted back to a form that existed
before and the configure tests handle it correctly.

Change-Id: If1fd9d27f795dee4b5aa2152dd09e0540d643a69
Reviewed-on: http://gerrit.openafs.org/11455
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: D Brashear <shadow@your-file-system.com>