bcachefs: Don't aggressively discard the journal

We frequently use 'bcachefs list_journal -a' for debugging, as it
provides a record of all btree transactions, and a history of what
happened.

But it's not so useful if we immediately discard journal buckets right
after they're no longer dirty.

This tweaks journal reclaim to only discard when we're low on space,
keeping the journal mostly un-discarded.

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