netfilter: nf_osf: avoid passing pointer to local var

gcc-10 points out that a code path exists where a pointer to a stack
variable may be passed back to the caller:

net/netfilter/nfnetlink_osf.c: In function 'nf_osf_hdr_ctx_init':
cc1: warning: function may return address of local variable [-Wreturn-local-addr]
net/netfilter/nfnetlink_osf.c:171:16: note: declared here
  171 |  struct tcphdr _tcph;
      |                ^~~~~

I am not sure whether this can happen in practice, but moving the
variable declaration into the callers avoids the problem.

Fixes: 31a9c292 ("netfilter: nf_osf: add struct nf_osf_hdr_ctx")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>