netlink: avoid underflow of groups bitset index
The subtraction is absolutely unnecessary and created an underflow with
926d2eadcb67. I don't see why it was useful before 926d2eadcb67 and even
before edf5608bfef3. The bitset addresses bits from zero to
NLP_MAX_GROUPS-1. Note that check of user supplied argument for
NETLINK_ADD_MEMBERSHIP and NETLINK_DROP_MEMBERSHIP socket options is
already correct !(optval >= NLP_MAX_GROUPS).