ipv6: flowlabel: do not disable BH where not needed

struct ip6_flowlabel are rcu managed, and call_rcu() is used
to delay fl_free_rcu() after RCU grace period.

There is no point disabling BH for pure RCU lookups.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>