bpf: silence uninitialized var warning in bpf_skb_net_grow

These three variables are set in one branch and used in another with
the same condition. But on some architectures they still generate
compiler warnings of the kind:

  warning: 'inner_trans' may be used uninitialized in this function [-Wmaybe-uninitialized]

Silence these false positives. Use the straightforward approach to
always initialize them, if a bit superfluous.

Fixes: 868d5235 ("bpf: add bpf_skb_adjust_room encap flags")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>