Fix input size computation in SetSPrefs pioctl

Fix input size computation in SetSPrefs pioctl

Patchset 718f85a8b69a78ac77beb5c8471af20657be2a53 contained
a small typo that prevents the SetSPrefs pioctl processing from
functioning in all cases.  fs setserverprefs continues to work
for non-DB preference lists because fs.c pokeServers() attempts
to try the old SetSPrefs33 pioctl in the non-DB server case.

Reviewed-on: http://gerrit.openafs.org/5465
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 552d59186b135af55eeaa6136a9cbb8965dd2f04)

Change-Id: I2226334c558bdc4d24e17d32751154f99f2a53a6
Reviewed-on: http://gerrit.openafs.org/5566
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>