amd64: fix get_user() on bitwise

We really need force-cast when converting to final result type;
unsigned long can be silently converted to integer types and
to pointers, but not to bitwise.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>