Time stats library (from bcache/bcachefs) & generic latency instrumentation

[Kent Overstreet]

Hey Coly, this is a fragment from a larger patch series - I wanted to send it
out to you now for review because it changes the bcache sysfs interface for
time_stats.

This series takes the time_stats code from bcachefs (which has some additional
improvements), moves it to lib/ converts bcache to use it (otherwise we get
clashes with the time_stats name) - and uses it for some really cool new latency
instrumentation.

These patches won't apply/build, full patch series for any interested is here:

https://evilpiepirate.org/git/bcachefs.git/log/?h=alloc_tags