ipv6: prevent a possible race condition with lifetimes

ipv6: prevent a possible race condition with lifetimes

[ Upstream commit 6c0d8833a605e195ae219b5042577ce52bf71fff ]

valid_lft, prefered_lft and tstamp are always accessed under the lock
"lock" in other places. Reading these without taking the lock may result
in inconsistencies regarding the calculation of the valid and preferred
variables since decisions are taken on these fields for those variables.

Signed-off-by: Niels Dossche <dossche.niels@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Niels Dossche <niels.dossche@ugent.be>
Link: https://lore.kernel.org/r/20220223131954.6570-1-niels.dossche@ugent.be
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>