netfilter: nfnetlink_queue: Initialize ctx to avoid memory allocation error

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nfnetlink_queue: Initialize ctx to avoid memory allocation error

It is possible that ctx in nfqnl_build_packet_message() could be used
before it is properly initialize, which is only initialized
by nfqnl_get_sk_secctx().

This patch corrects this problem by initializing the lsmctx to a safe
value when it is declared.

This is similar to the commit 35fcac7a7c25
("audit: Initialize lsmctx to avoid memory allocation error").