xfrm: Convert xfrm_get_tos() to dscp_t.

Return a dscp_t variable to prepare for the future conversion of
xfrm_bundle_create() to dscp_t.

While there, rename the function "xfrm_get_dscp", to align its name
with the new return type.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>