bcachefs: Convert compression_stats to for_each_btree_key2

for_each_btree_key2() runs each loop iteration in a btree transaction,
and thus does not cause SRCU lock hold time problems.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/sysfs.c

@@ -276,8 +276,8 @@ static int bch2_compression_stats_to_text(struct printbuf *out, struct bch_fs *c
 		if (!btree_type_has_ptrs(id))
 			continue;
 
-		for_each_btree_key(trans, iter, id, POS_MIN,
-				   BTREE_ITER_ALL_SNAPSHOTS, k, ret) {
+		ret = for_each_btree_key2(trans, iter, id, POS_MIN,
+					  BTREE_ITER_ALL_SNAPSHOTS, k, ({
 			struct bkey_ptrs_c ptrs = bch2_bkey_ptrs_c(k);
 			const union bch_extent_entry *entry;
 			struct extent_ptr_decoded p;
@@ -309,8 +309,8 @@ static int bch2_compression_stats_to_text(struct printbuf *out, struct bch_fs *c
 				nr_uncompressed_extents++;
 			else if (compressed)
 				nr_compressed_extents++;
-		}
-		bch2_trans_iter_exit(trans, &iter);
+			0;
+		}));
 	}
 
 	bch2_trans_put(trans);