[PATCH] ide: add sanity checking to ide taskfile ioctl

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: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>