ipv6: remove unnecessary inet6_sk check

np is already assigned in the variable declaration of ping_v6_sendmsg.
At this point, we have already dereferenced np several times, so the
NULL check is also redundant.

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>