sit: fix off-by-one in ipip6_tunnel_get_prl

When requesting all prl entries (kprl.addr == INADDR_ANY) and there are
more prl entries than there is space passed from userspace, the existing
code would always copy cmax+1 entries, which is more than can be handled.

This patch makes the kernel copy only exactly cmax entries.

Signed-off-by: Sascha Hlusiak <contact@saschahlusiak.de>
Acked-By: Fred L. Templin <Fred.L.Templin@boeing.com>
Signed-off-by: David S. Miller <davem@davemloft.net>