s390/qeth: streamline l3_fill_header()

Consolidate the cast type translation, move the passthru path out of
the RCU-guarded section, and use the appropriate rtable helpers when
determining the next-hop address.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>