Don't pass meaningless file handles to block device ioctls.

The recent raw IO ioctl-passthrough fix started passing the raw file
handle into the block device ioctl handler.  That's unlikely to be
useful, as the file handle is actually open on a character-mode raw
device, not a block device, so dereferencing it is not going to yield
useful results to a block device ioctl handler.

Previously we just passed NULL; also not a value that can usefully
be dereferenced, but at least if it does happen, we'll oops instead of
silently pretending that the file is a block device, so NULL is the more
defensive option here.  This patch reverts to that behaviour.

Noticed by Al Viro.
Signed-off-by: NStephen Tweedie <sct@redhat.com>
Acked-by: NAl Viro <viro@parcelfarce.linux.theplanet.co.uk>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>