bcachefs: Add a persistent counter for bucket discards

Like the previous patch for bucket invalidates, add another counter for
a core allocator path.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/alloc_background.c

@@ -1052,6 +1052,7 @@ static void bch2_do_discards_work(struct work_struct *work)
 		if (ret)
 			break;
 
+		this_cpu_inc(c->counters[BCH_COUNTER_bucket_discard]);
 		discarded++;
 	}
 	bch2_trans_iter_exit(&trans, &iter);

fs/bcachefs/bcachefs_format.h

@@ -1330,7 +1330,8 @@ struct bch_sb_field_disk_groups {
 	x(io_read,		0)			\
 	x(io_write,		1)			\
 	x(io_move,		2)			\
-	x(bucket_invalidate,	3)
+	x(bucket_invalidate,	3)			\
+	x(bucket_discard,	4)
 
 enum bch_persistent_counters {
 #define x(t, n, ...) BCH_COUNTER_##t,