[IGMP]: Fix IGMPV3_EXP() normalization bit shift value.

The IGMPV3_EXP() macro doesn't correctly shift the normalization bit, so
time-out values are longer than they should be.

Thanks to Dirk Ooms for finding the problem in IGMPv3 - MLDv2 had a
similar problem that was already fixed a year ago. :-(

Signed-off-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>