include/atomic: make 32-bit atomic64_t unsigned

This fixes

In file included from test/perf_counters.cc:19:0:
./common/perf_counters.h: In member function ‘std::pair PerfCounters::perf_counter_data_any_d::read_avg() const’:
warning: ./common/perf_counters.h:156:36: comparison between signed and unsigned integer expressions [-Wsign-compare]
} while (avgcount2.read() != count);
^

Signed-off-by: Sage Weil <sage@inktank.com>