bcachefs: Fix bch2_extent_ptr_durability()

We were looking up the wrong entry in the stripes radix tree.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/extents.c

@@ -79,7 +79,7 @@ static unsigned bch2_extent_ptr_durability(struct bch_fs *c,
 
 	for (i = 0; i < p.ec_nr; i++) {
 		struct stripe *s =
-			genradix_ptr(&c->stripes[0], p.idx);
+			genradix_ptr(&c->stripes[0], p.ec[i].idx);
 
 		if (WARN_ON(!s))
 			continue;