Don't define our own ioctls unless we have to.

Don't define our own ioctls unless we have to.

The way it was, BLKGETSIZE64 wound up being 0x80041272 instead of the
0x80081272 it should be on -m32 builds.  4 isn't the size of 64 bits, so
that results in integer overflow into who knows what space on the stack,
and when we compare it to our gpt header, it's also just plain wrong.

This is all madness, of course, the kernel has exported all of this
stuff since the ancient past.  Use that instead.

Signed-off-by: Peter Jones <pjones@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Tested-by: Michael Tokarev <mjt@tls.msk.ru>