[IPV4]: Remove unnecessary check for opt->is_data in ip_options_compile.

There is the only way to reach ip_options compile with opt != NULL:

ip_options_get_finish
    opt->is_data = 1;
    ip_options_compile(opt, NULL)

So, checking for is_data inside opt != NULL branch is not needed.

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