inet: properly align icsk_ca_priv

tcp_illinois and upcoming tcp_cdg require 64bit alignment of
icsk_ca_priv

x86 does not care, but other architectures might.

Fixes: 05cbc0db

 ("ipv4: Create probe timer for tcp PMTU as per RFC4821")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Fan Du <fan.du@intel.com>
Acked-by: Fan Du <fan.du@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>