With the seqlock, we either have to use atomics to remain
within defined behaviour (and note that 64-bit atomics aren't
always guaranteed to compile, irrespective of __nocheck), or
drop the atomics and be in undefined behaviour territory.

Fix it by dropping the seqlock and using atomic64 accessors.
This will limit scalability when !CONFIG_ATOMIC64, but those
machines (1) don't have many users and (2) are unlikely to
have many cores.

- With CONFIG_ATOMIC64:
$ tests/atomic_add-bench -n 1 -m -p
 Throughput:         13.00 Mops/s

- Forcing !CONFIG_ATOMIC64:
$ tests/atomic_add-bench -n 1 -m -p
 Throughput:         10.89 Mops/s

Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <20180910232752.31565-5-cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>