xfrm: policy: xfrm_get_tos cannot fail

The comment makes it look like get_tos() is used to validate something,
but it turns out the comment was about xfrm_find_bundle() which got removed
years ago.

xfrm_get_tos will return either the tos (ipv4) or 0 (ipv6).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>