net: ipv6: fix alignment errors on ARM

Commands "ping6" and "tftpboot ... -ipv6" did not work on ARM because
machine code expects 4-byte alignment and some structures from net6.h
are not aligned in memory.

Fix by adding __packed, since it is already used in this file.

Signed-off-by: Sergei Antonov <saproj@gmail.com>
Reviewed-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>