ipv6: Set cfg.ifa_flags before device lookup in inet6_rtm_newaddr().

ipv6: Set cfg.ifa_flags before device lookup in inet6_rtm_newaddr().

We will convert inet6_rtm_newaddr() to per-netns RTNL.

Except for IFA_F_OPTIMISTIC, cfg.ifa_flags can be set before
__dev_get_by_index().

Let's move ifa_flags setup before __dev_get_by_index() so that
we can set ifa_flags without RTNL.

Also, now it's moved before tb[IFA_CACHEINFO] in preparing for
the next patch.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250115080608.28127-9-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>