net-next: gro: Fix use of skb_gro_header_slow

In the cited commit, the function ipv6_gro_receive was accidentally
changed to use skb_gro_header_slow, without attempting the fast path.
Fix it.

Fixes: 35ffb665 ("net: gro: skb_gro_header helper function")
Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
Link: https://lore.kernel.org/r/20220911184835.GA105063@debian


Signed-off-by: Paolo Abeni <pabeni@redhat.com>