bcachefs: Fix "journal stuck" during recovery

If we crash when the journal pin fifo is completely full - i.e. we're
at the maximum number of dirty journal entries - that may put us in a
sticky situation in recovery, as journal replay will need to be able to
open new journal entries in order to get going.

bch2_fs_journal_start() already had provisions for resizing the journal
pin fifo if needed, but it needs a fudge factor to ensure there's room
for journal replay.

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