bonding: remove bond_resend_igmp_join_requests read_unlock leftover

After commit 4aa5dee4 ("net: convert resend IGMP to notifier event") we
have 1 read_unlock in bond_resend_igmp_join_requests which isn't paired
with a read_lock because it's removed by that commit.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Reviewed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>