bcachefs: enumerated_ref.c

bcachefs: enumerated_ref.c

Factor out the debug code for rw filesystem refs into a small library.

In release mode an enumerated ref is a normal percpu refcount, but in
debug mode all enumerated users of the ref get their own atomic_long_t
ref - making it much easier to chase down refcount usage bugs for when a
refcount has many users.

For debugging, we have enumerated_ref_to_text(), which prints the
current value of each different user.

Additionally, in debug mode enumerated_ref_stop() has a 10 second
timeout, after which it will dump outstanding refcounts.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>