Several functions receive pointers to u8, char or sk_buff but do not
modify the contents so make them const.  This allows doing the same for
local variables and in total makes the code a little bit safer.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>