wifi: mac80211: fix race in mesh sequence number assignment

Since the sequence number is shared across different tx queues, it needs
to be atomic in order to avoid accidental duplicate assignment

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20230314095956.62085-2-nbd@nbd.name


Signed-off-by: Johannes Berg <johannes.berg@intel.com>