bcachefs: Kill journal pre-reservations

This deletes the complicated and somewhat expensive journal
pre-reservation machinery in favor of just using journal watermarks:
when the journal is more than half full, we run journal reclaim more
aggressively, and when the journal is more than 3/4s full we only allow
journal reclaim to get new journal reservations.

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