ipv6: fix a typo in fib6_rule_lookup()

Yi Ren reported an issue discovered by syzkaller, and bisected
to the cited commit.

Many thanks to Yi, this trivial patch does not reflect the patient
work that has been done.

Fixes: d64a1f574a29 ("ipv6: honor RT6_LOOKUP_F_DST_NOREF in rule lookup logic")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Wei Wang <weiwan@google.com>
Bisected-and-reported-by: Yi Ren <c4tren@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
