bcachefs: Fix -Wself-assign

Fixes the following observed error reported by Nathan on IRC.

  fs/bcachefs/io_misc.c:467:6: error: explicitly assigning value of
  variable of type 'int' to itself [-Werror,-Wself-assign]
    467 |         ret = ret;
        |         ~~~ ^ ~~~

Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/io_misc.c

@@ -462,9 +462,9 @@ btree_err:
 				bch2_logged_op_update(trans, &op->k_i));
 	}
 
-	fallthrough;
+	break;
 case LOGGED_OP_FINSERT_finish:
-	ret = ret;
+	break;
 	}
 err:
 	bch2_logged_op_finish(trans, op_k);