geneve: Get rid of is_all_zero(), streamline is_tnl_info_zero()

No need to re-invent memchr_inv() with !is_all_zero(). While at
it, replace conditional and return clauses with a single return
clause in is_tnl_info_zero().

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>