This problem occurs only if NET_MPSAFE on.
ifconfig destroy side:
kernel entry point is ifioctl => if_clone_destroy.
pr_purgeif() acquires softnet_lock, and then ifa_remove() calls
pserialize_perform() holding softnet_lock.
ifconfig side:
kernel entry point is socreate.
pr_attach()(udp_attach_wrapper()) calls sosetlock(). In this call path,
sosetlock() try to acquire softnet_lock.
These can cause dead lock.
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