net: support 64bit rates for getsockopt(SO_MAX_PACING_RATE)

For legacy applications using 32bit variable, SO_MAX_PACING_RATE
has to cap the returned value to 0xFFFFFFFF, meaning that
rates above 34.35 Gbit are capped.

This patch allows applications to read socket pacing rate
at full resolution, if they provide a 64bit variable to store it,
and the kernel is 64bit.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>