net: tcp_memcontrol: simplify linkage between socket and page counter

There won't be any separate counters for socket memory consumed by
protocols other than TCP in the future.  Remove the indirection and link
sockets directly to their owning memory cgroup.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>