socket: sk_filter minor cleanups

Some minor style cleanups:
  * Move __KERNEL__ definitions to one place in filter.h
  * Use const for sk_filter_len
  * Line wrapping
  * Put EXPORT_SYMBOL next to function definition

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>