net80211: remove node scan lock / generation number + fix few LORs

[avos]

Summary

Drop scan generation number and node table scan lock - the only place where ni_scangen is checked is in ieee80211_timeout_stations() (and it is used to prevent duplicate checking of the same node); node scan lock protects only this variable + node table scan generation number.

This will fix (at least) next LOR (hostap mode):
lock order reversal:
1st 0xc175f84c urtwm0_scan_loc (urtwm0_scan_loc) @ /usr/src/sys/modules/wlan/../../net80211/ieee80211_node.c:2019
2nd 0xc175e018 urtwm0_com_lock (urtwm0_com_lock) @ /usr/src/sys/modules/wlan/../../net80211/ieee80211_node.c:2693
stack backtrace:
#0 0xa070d1c5 at witness_debugger+0x75
#1 0xa070d0f6 at witness_checkorder+0xd46
#2 0xa0694cce at __mtx_lock_flags+0x9e
#3 0xb03ad9ef at ieee80211_node_leave+0x12f
#4 0xb03afd13 at ieee80211_timeout_stations+0x483
#5 0xb03aa1c2 at ieee80211_node_timeout+0x42
#6 0xa06c6fa1 at softclock_call_cc+0x1e1
#7 0xa06c7518 at softclock+0xc8
#8 0xa06789ae at intr_event_execute_handlers+0x8e
#9 0xa0678fa0 at ithread_loop+0x90
#10 0xa0675fbe at fork_exit+0x7e
#11 0xa08af910 at fork_trampoline+0x8

[avos]

In addition to the above:

• switch to ieee80211_iterate_nodes();
• do not assert that node table lock is held, while calling node_age(); that's not really needed (there are no resources, which can be protected by this lock) + this fixes LOR/deadlock between ieee80211_timeout_stations() and ieee80211_set_tim() (easy to reproduce in HOSTAP mode while sending something to an STA with enabled power management).