ipv6: Move lifetime validation to inet6_rtm_newaddr().

ipv6: Move lifetime validation to inet6_rtm_newaddr().

inet6_addr_add() and inet6_addr_modify() have the same code to validate
IPv6 lifetime that is done under RTNL.

Let's factorise it out to inet6_rtm_newaddr() so that we can validate
the lifetime without RTNL later.

Note that inet6_addr_add() is called from addrconf_add_ifaddr(), but the
lifetime is INFINITY_LIFE_TIME in the path, so expires and flags are 0.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250115080608.28127-10-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>