[IPV6] __inet6_csk_dst_store(): fix check-after-use

The Coverity checker spotted that we have already oops'ed if "dst" was
NULL.

Since "dst" being NULL doesn't seem to be possible at this point this
patch removes the NULL check.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Acked-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Acked-by: Noriaki TAKAMIYA <takamiya@po.ntts.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>