net: remove padding from struct socket on 64bit & increase objects/cache

remove padding from struct socket reducing its size by 8 bytes.
    
This allows more objects/cache in sock_inode_cache
12 objects/cache when cacheline size is 128 (generic x86_64)
    
Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>