ipv4: add ip_sock_set_tos

Add a helper to directly set the IP_TOS sockopt from kernel space without
going through a fake uaccess.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: David S. Miller <davem@davemloft.net>