NFSD: Support AF_INET6 in svc_addsock() function

Relax the address family check at the top of svc_addsock() to allow AF_INET6
listener sockets to be specified via /proc/fs/nfsd/portlist.

Signed-off-by: Aime Le Rouzic <aime.le-rouzic@bull.net>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>