zswap: ignore debugfs_create_dir() return value

When calling debugfs functions, there is no need to ever check the
return value.  The function can work or not, but the code logic should
never do something different based on this.

Cc: Seth Jennings <sjenning@redhat.com>
Cc: linux-mm@kvack.org
Acked-by: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>