MINOR: freq_ctr: add opportunistic versions of swrate_add()

MINOR: freq_ctr: add opportunistic versions of swrate_add()

Some uses of swrate_add() only consist in getting a rough estimate of
a frequency. There are cases where speed matters more than accuracy
(e.g. pools). For such use cases, let's just stop looping on the CAS,
if the update fails, another thread is already providing input, and
it's not dramatic to lose the race. All these functions are now
suffixed with "_opportunistic".