revokefs: Fix off-by-one array size in fuse implementation

The iovec write_vecs[] array on the stack was just 2 element, but we
sometimes use 3 elements of it. This was causing random crashes due to
corrupting the stack.

Closes: #3110
Approved by: alexlarsson

revokefs/writer.c

@@ -60,7 +60,7 @@ do_request (int writer_socket,
  size_t request_size;
  size_t response_max_size;
  ssize_t written_size, read_size;
- struct iovec write_vecs[2] = {};
+ struct iovec write_vecs[3] = {};
  int n_write_vecs = 0;
  struct iovec read_vecs[2] = {};
  int n_read_vecs = 0;