nf: xt_socket: Fix out-of-bounds in xt_socket_lookup_slow_v{4,6}

skb_header_pointer will copy data into the buffer if data is spread
across pages, else share pointer within skb->data. In
xt_socket_lookup_slow_v4, data is copied into udphdr, and later
th->doff is accessed, causing a out-of-bounds. This
access will only work if the data is not split across pages.

Copy data into tcphdr buffer for TCP packets instead.

Change-Id: Ifd6e15ece27fcf5bd02ae17571ab43f6df3ceb21
Fixes: a583636a ("inet: refactor inet[6]_lookup functions to take skb")
Signed-off-by: Tejaswi Tanikella <tejaswit@codeaurora.org>