bcachefs: Eliminate padding in move_bucket_key

We appear to be tripping over a compiler/kmsan bug with padding fields -
this is an easy workaround.

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

Author: Kent Overstreet

fs/bcachefs/move_types.h

@@ -32,7 +32,7 @@ struct bch_move_stats {
 
 struct move_bucket_key {
 	struct bpos		bucket;
-	u8			gen;
+	unsigned		gen;
 };
 
 struct move_bucket {