net: ipv6: network protocol structures should be packed

The structure icmp6_ra_prefix_info needs to be packed because it is read
from a network stream.

Signed-off-by: Ehsan Mohandesi <emohandesi@linux.microsoft.com>
Reviewed-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>