bcachefs: Fix gc updating stripes info

The primary stripes radix tree can be sparse, which was causing an
assertion to pop because the one use for gc isn't. Fix this by changing
the algorithm to copy between the two radix trees.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_gc.c

@@ -580,7 +580,7 @@ static int bch2_gc_done(struct bch_fs *c,
 		if (verify)						\
 			fsck_err(c, "stripe %zu has wrong "_msg		\
 				": got %u, should be %u",		\
-				dst_iter.pos, ##__VA_ARGS__,		\
+				iter.pos, ##__VA_ARGS__,		\
 				dst->_f, src->_f);			\
 		dst->_f = src->_f;					\
 		dst->dirty = true;					\
@@ -603,13 +603,11 @@ static int bch2_gc_done(struct bch_fs *c,
 	copy_field(_f, "fs has wrong " _msg, ##__VA_ARGS__)
 
 	if (!metadata_only) {
-		struct genradix_iter dst_iter = genradix_iter_init(&c->stripes[0], 0);
-		struct genradix_iter src_iter = genradix_iter_init(&c->stripes[1], 0);
+		struct genradix_iter iter = genradix_iter_init(&c->stripes[1], 0);
 		struct stripe *dst, *src;
 
-		while ((dst = genradix_iter_peek(&dst_iter, &c->stripes[0])) &&
-		       (src = genradix_iter_peek(&src_iter, &c->stripes[1]))) {
-			BUG_ON(src_iter.pos != dst_iter.pos);
+		while ((src = genradix_iter_peek(&iter, &c->stripes[1]))) {
+			dst = genradix_ptr_alloc(&c->stripes[0], iter.pos, GFP_KERNEL);
 
 			copy_stripe_field(alive,	"alive");
 			copy_stripe_field(sectors,	"sectors");
@@ -623,8 +621,7 @@ static int bch2_gc_done(struct bch_fs *c,
 				copy_stripe_field(block_sectors[i],
 						  "block_sectors[%u]", i);
 
-			genradix_iter_advance(&dst_iter, &c->stripes[0]);
-			genradix_iter_advance(&src_iter, &c->stripes[1]);
+			genradix_iter_advance(&iter, &c->stripes[1]);
 		}
 	}