ovs-atomic-c++.h: Fix for 64 bit atomics.

ovs-atomic-c++.h: Fix for 64 bit atomics.

Commit e981a45a6cae4 (ovs-atomic: Add 64 bit apis.)
added a few 64 bit apis (e.g: atomic_count_inc64).  For C++,
this invokes std::atomic_fetch_*_explicit() functions in
lib/ovs-atomic-c++.h.

The function overloading for 64 bit function fails without
specifiying something like: std::atomic_fetch_*_explicit<std::uint64_t>().
But it looks tricky to do this with macros.

This patch tries to fix the compilation failures by calling atomic
functions on the variables itself.

Signed-off-by: Gurucharan Shetty <guru@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>