[NET]: Re-fix of doc-comment in sock.h

Restoring old, correct comment for sk_filter_release, moving it to
where it should actually be, and changing new comment into proper
comment for sk_filter_rcu_free, where it actually makes sense.

The original fix submitted for this on Oct 23 mistakenly documented
the wrong function.

Signed-off-by: Paul Bonser <misterpib@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>