vxlan: Read jiffies once when updating FDB 'used' time

Avoid two volatile reads in the data path. Instead, read jiffies once
and only if an FDB entry was found.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20250204145549.1216254-3-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>