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.

Change-Id: I2e06dccb51c53ba312418fb5f51be3d621a2004c
Reviewed-on: http://gerrit.openafs.org/5465
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>