bcachefs: rebalance writes use BCH_WRITE_ONLY_SPECIFIED_DEVS

this was an oversight: rebalance is moving data to a specific device, so
we don't want it falling back to the full filesystem
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_foreground.c

@@ -1389,8 +1389,6 @@ int bch2_alloc_sectors_start_trans(struct btree_trans *trans,
 	if (!IS_ENABLED(CONFIG_BCACHEFS_ERASURE_CODING))
 		erasure_code = false;
 
-	BUG_ON(flags & BCH_WRITE_ONLY_SPECIFIED_DEVS);
-
 	BUG_ON(!nr_replicas || !nr_replicas_required);
 retry:
 	ptrs.nr		= 0;

fs/bcachefs/rebalance.c

@@ -13,6 +13,7 @@
 #include "errcode.h"
 #include "error.h"
 #include "inode.h"
+#include "io_write.h"
 #include "move.h"
 #include "rebalance.h"
 #include "subvolume.h"
@@ -156,6 +157,7 @@ static struct bkey_s_c next_rebalance_extent(struct btree_trans *trans,
 	data_opts->rewrite_ptrs		=
 		bch2_bkey_ptrs_need_rebalance(c, k, r->target, r->compression);
 	data_opts->target		= r->target;
+	data_opts->write_flags		|= BCH_WRITE_ONLY_SPECIFIED_DEVS;
 
 	if (!data_opts->rewrite_ptrs) {
 		/*
@@ -263,6 +265,7 @@ static bool rebalance_pred(struct bch_fs *c, void *arg,
 
 	data_opts->rewrite_ptrs		= bch2_bkey_ptrs_need_rebalance(c, k, target, compression);
 	data_opts->target		= target;
+	data_opts->write_flags		|= BCH_WRITE_ONLY_SPECIFIED_DEVS;
 	return data_opts->rewrite_ptrs != 0;
 }