net: sk_update_clone is only used in net/core/sock.c

so move it there.  Fixes build errors when CONFIG_INET is not defined:

In file included from include/linux/tcp.h:211:0,
                 from include/linux/ipv6.h:221,
                 from include/net/ipv6.h:16,
                 from include/linux/sunrpc/clnt.h:26,
                 from include/linux/nfs_fs.h:50,
                 from init/do_mounts.c:20:
include/net/sock.h: In function 'sk_update_clone':
include/net/sock.h:1109:3: error: implicit declaration of function 'sock_update_memcg' [-Werror=implicit-function-declaration]

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>