remove unnecessary casts from uintptr_t

remove unnecessary casts from uintptr_t

uintptr_t, or unsigned long which is equivalent on Linux I32LP64 systems,
is an unsigned type and there is no need to further cast to __u64 which is
another unsigned integer type; widening casts from unsigned integers
zero-extend the value.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>