bcachefs: Be more conservation about journal pre-reservations

bcachefs: Be more conservation about journal pre-reservations

 - Try to always keep 1/8th of the journal free, on top of
   pre-reservations
 - Move the check for whether the journal is stuck to
   bch2_journal_space_available, and make it only fire when there aren't
   any journal writes in flight (that might free up space by updating
   last_seq)

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