VMCI: Fixup atomic64_t abuse

The VMCI driver is abusing atomic64_t and atomic_t, there is no actual
atomic RmW operations around.

Rewrite the code to use a regular u64 with READ_ONCE() and
WRITE_ONCE() and a cast to 'unsigned long'. This fully preserves
whatever broken there was (it's not endian-safe for starters, and also
looks to be missing ordering).

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>