ipv6: keep existing flags when setting IFA_F_OPTIMISTIC

ipv6: keep existing flags when setting IFA_F_OPTIMISTIC

Commit 64236f3f3d74 ("ipv6: introduce IFA_F_STABLE_PRIVACY flag")
failed to update the setting of the IFA_F_OPTIMISTIC flag, causing
the IFA_F_STABLE_PRIVACY flag to be lost if IFA_F_OPTIMISTIC is set.

Cc: Erik Kline <ek@google.com>
Cc: Fernando Gont <fgont@si6networks.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: YOSHIFUJI Hideaki/吉藤英明 <hideaki.yoshifuji@miraclelinux.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Fixes: 64236f3f3d74 ("ipv6: introduce IFA_F_STABLE_PRIVACY flag")
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>