net: Use __u32 in uapi net_stamp.h

We are not supposed to use u32 in uapi, so change the flags member of
struct sock_txtime from u32 to __u32 instead.

Fixes: 80b14dee ("net: Add a new socket option for a future transmit time")
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>