ipv6/addrconf: fix timing bug in tempaddr regen

The addrconf_verify_rtnl() function uses a big if/elseif/elseif/... block
to categorize each address by what type of attention it needs.  An
about-to-expire (RFC 4941) temporary address is one such category, but the
previous elseif branch catches addresses that have already run out their
prefered_lft.  This means that if addrconf_verify_rtnl() fails to run in
the necessary time window (i.e. REGEN_ADVANCE time units before the end of
the prefered_lft), the temporary address will never be regenerated, and no
temporary addresses will be available until each one's valid_lft runs out
and manage_tempaddrs() begins anew.

Fix this by moving the entire temporary address regeneration case out of
that block.  That block is supposed to implement the "destructive" part of
an address's lifecycle, and regenerating a fresh temporary address is not,
semantically speaking, actually tied to any particular lifecycle stage.
The age test is also changed from `age >= prefered_lft - regen_advance`
to `age + regen_advance >= prefered_lft` instead, to ensure no underflow
occurs if the system administrator increases the regen_advance to a value
greater than the already-set prefered_lft.

Note that this does not fix the problem of addrconf_verify_rtnl() sometimes
not running in time, resulting in the race condition described in RFC 4941
section 3.4 - it only ensures that the address is regenerated.  Fixing THAT
problem may require either using jiffies instead of seconds for all time
arithmetic here, or always rounding up when regen_advance is converted to
seconds.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Link: https://lore.kernel.org/r/20220623181103.7033-1-CFSworks@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>