udp: use indirect call wrappers for GRO socket lookup

This avoids another indirect call for UDP GRO. Again, the test
for the IPv6 variant is performed first.

v1 -> v2:
 - adapted to INDIRECT_CALL_ changes

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>