Without this the user can feed in bogus values and get very bogus
results. Security impact is minimal as this ioctl isn't available to
unpriviledged processes anyway.

Reported to the l/k list and found with an auditing tool.
Signed-off-by: NAlan Cox <alan@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>