bcachefs: Add a fallback when journal_keys doesn't fit in ram

bcachefs: Add a fallback when journal_keys doesn't fit in ram

We may end up in a situation where allocating the buffer for the sorted
journal_keys fails - but it would likely succeed, post compaction where
we drop duplicates.

We've had reports of this allocation failing, so this adds a slowpath to
do the compaction incrementally.

This is only a band-aid fix; we need to look at limiting the number of
keys in the journal based on the amount of system RAM.

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