[IPV4]: skb->dst can't be NULL in ip_options_echo.

ip_options_echo is called on the packet input path after the initial
routing. The dst entry on the packet is cleared only in the several
very specific places and immidiately assigned back (may be new).

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>