drivers: staging: rtl8192u: Fix deadlock in ieee80211_beacons_stop()

In the Linux kernel, the following vulnerability has been resolved:

drivers: staging: rtl8192u: Fix deadlock in ieee80211_beacons_stop()

There is a deadlock in ieee80211_beacons_stop(), which is shown below:

(Thread 1) | (Thread 2)

| ieee80211_send_beacon()

ieee80211_beacons_stop() | mod_timer()

spin_lock_irqsave() //(1) | (wait a time)

... | ieee80211_send_beacon_cb()

del_timer_sync() | spin_lock_irqsave() //(2)

(wait timer to stop) | ...

We hold ieee->beacon_lock in position (1) of thread 1 and use

del_timer_sync() to wait timer to stop, but timer handler

also need ieee->beacon_lock in position (2) of thread 2.

As a result, ieee80211_beacons_stop() will block forever.

This patch extracts del_timer_sync() from the protection of

spin_lock_irqsave(), which could let timer handler to obtain

the needed lock.