bcachefs: Fix an unhandled transaction restart error

This is a bit awkward: we're passing around a btree_trans, but we're not
in a context where transaction restarts are handled - we should try to
come up with a better way to denote situations like this.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/move.c

@@ -713,6 +713,11 @@ int __bch2_evacuate_bucket(struct btree_trans *trans,
 
 	bch2_bkey_buf_init(&sk);
 
+	/*
+	 * We're not run in a context that handles transaction restarts:
+	 */
+	bch2_trans_begin(trans);
+
 	bch2_trans_iter_init(trans, &iter, BTREE_ID_alloc,
 			     bucket, BTREE_ITER_CACHED);
 	ret = lockrestart_do(trans,