bcachefs: Fix triggering of discard by the journal path

It becomes possible to do discards after a journal flush, which
naturally the journal code is reponsible for.

A prior refactoring seems to have broken this - which went unnoticed
because the foreground allocator path can also trigger discards.

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

fs/bcachefs/journal_io.c

@@ -1767,6 +1767,7 @@ static CLOSURE_CALLBACK(journal_write_done)
 
 			closure_wake_up(&c->freelist_wait);
 			bch2_reset_alloc_cursors(c);
+			do_discards = true;
 		}
 
 		j->seq_ondisk = seq;