net: diag: split inet_diag_dump_one_icsk into two

Currently, inet_diag_dump_one_icsk finds a socket and then dumps
its information to userspace. Split it into a part that finds the
socket and a part that dumps the information.

[Backport of net-next b613f56e]

Change-Id: I6b570c35a4928ea072de3e31f9f3ca89668a5244
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>