netns: constify net_hash_mix() and various callers

const qualifiers ease code review by making clear
which objects are not written in a function.

Change-Id: I32cbf7b5c97fc4139313abfc13ec6b54bc14ddf8
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 6eada0110c8984477f5f1e57a0b7f7b2fc841e30
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Matt Wagantall <mattw@codeaurora.org>