inet_diag: remove duplicate code from inet_twsk_diag_dump()

timewait sockets now share a common base with established sockets.

inet_twsk_diag_dump() can use inet_diag_bc_sk() instead of duplicating
code, granted that inet_diag_bc_sk() does proper userlocks
initialization.

twsk_build_assert() will catch any future changes that could break
the assumptions.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>