bcachefs: Improve journal pin flushing

Running the preempt tiering tests with a lower than normal journal
reclaim delay turned up a shutdown hang - a lost wakeup, caused because
flushing a journal pin (e.g. key cache/write buffer) can generate a new
journal pin.

The "simple" fix of adding the correct wakeup didn't work because of
ordering issues; if we flush btree node pins too aggressively before
other pins have completed, we end up spinning where each flush iteration
generates new work.

So to fix this correctly:
- The list of flushed journal pins is now broken out by type, so that
  we can wait for key cache/write buffer pin flushing to complete
  before flushing dirty btree nodes

- A new closure_waitlist is added for bch2_journal_flush_pins; this one
  is only used under or when we're taking the journal lock, so it's
  pretty cheap to add rigorously correct wakeups to journal_pin_set()
  and journal_pin_drop().

Additionally, bch2_journal_seq_pins_to_text() is moved to
journal_reclaim.c, where it belongs, along with a bit of other small
renaming and refactoring.

Besides fixing the hang, the better ordering between key cache/write
buffer flushing and btree node flushing should help or fix the "unmount
taking excessively long" a few users have been noticing.

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