ipv6: mcast: fix unsolicited report interval after receiving querys

After recieving MLD querys, we update idev->mc_maxdelay with max_delay
from query header. This make the later unsolicited reports have the same
interval with mc_maxdelay, which means we may send unsolicited reports with
long interval time instead of default configured interval time.

Also as we will not call ipv6_mc_reset() after device up. This issue will
be there even after leave the group and join other groups.

Fixes: fc4eba58 ("ipv6: make unsolicited report intervals configurable for mld")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>