percpu_counter: add a cmpxchg-based _add_batch variant

percpu_counter: add a cmpxchg-based _add_batch variant

Interrupt disable/enable trips are quite expensive on x86-64 compared to a
mere cmpxchg (note: no lock prefix!) and percpu counters are used quite
often.

With this change I get a bump of 1% ops/s for negative path lookups,
plugged into will-it-scale:

void testcase(unsigned long long *iterations, unsigned long nr)
{
        while (1) {
                int fd = open("/tmp/nonexistent", O_RDONLY);
                assert(fd == -1);

                (*iterations)++;
        }
}

The win would be higher if it was not for other slowdowns, but one has
to start somewhere.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Mateusz Guzik <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Dennis Zhou <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>