There's a bug in addrconf_prefix_rcv() where it won't update the
preferred lifetime of an IPv6 address if the current valid lifetime
of the address is less than 2 hours (the minimum value in the RA).

For example, If I send a router advertisement with a prefix that
has valid lifetime = preferred lifetime = 2 hours we'll build
this address:

3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
    inet6 2001:1890:1109:a20:217:8ff:fe7d:4718/64 scope global dynamic
       valid_lft 7175sec preferred_lft 7175sec

If I then send the same prefix with valid lifetime = preferred
lifetime = 0 it will be ignored since the minimum valid lifetime
is 2 hours:

3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
    inet6 2001:1890:1109:a20:217:8ff:fe7d:4718/64 scope global dynamic
       valid_lft 7161sec preferred_lft 7161sec

But according to RFC 4862 we should always reset the preferred lifetime
even if the valid lifetime is invalid, which would cause the address
to immediately get deprecated.  So with this patch we'd see this:

5: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
    inet6 2001:1890:1109:a20:21f:29ff:fe5a:ef04/64 scope global deprecated dynamic
       valid_lft 7163sec preferred_lft 0sec

The comment winds-up being 5x the size of the code to fix the problem.

Update the preferred lifetime of IPv6 addresses derived from a prefix
info option in a router advertisement even if the valid lifetime in
the option is invalid, as specified in RFC 4862 Section 5.5.3e.  Fixes
an issue where an address will not immediately become deprecated.
Reported by Jens Rosenboom.
Signed-off-by: NBrian Haley <brian.haley@hp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>