netfilter: nf_flow_table_offload: Don't use offset uninitialized in flow_offload_port...

netfilter: nf_flow_table_offload: Don't use offset uninitialized in flow_offload_port_{d,s}nat

Clang warns (trimmed the second warning for brevity):

../net/netfilter/nf_flow_table_offload.c:342:2: warning: variable
'offset' is used uninitialized whenever switch default is taken
[-Wsometimes-uninitialized]
        default:
        ^~~~~~~
../net/netfilter/nf_flow_table_offload.c:346:57: note: uninitialized use
occurs here
        flow_offload_mangle(entry, flow_offload_l4proto(flow), offset,
                                                               ^~~~~~
../net/netfilter/nf_flow_table_offload.c:331:12: note: initialize the
variable 'offset' to silence this warning
        u32 offset;
                  ^
                   = 0

Match what was done in the flow_offload_ipv{4,6}_{d,s}nat functions and
just return in the default case, since port would also be uninitialized.

Fixes: c29f74e0df7a ("netfilter: nf_flow_table: hardware offload support")
Link: https://github.com/ClangBuiltLinux/linux/issues/780
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reported-by: kernelci.org bot <bot@kernelci.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>