There's only one handle that's likely to be open in a long lived way:
the tun registration handle. So we can force that closed automatically
when the device is about to close, if it's been improperly left open.
Other handles will indeed hold up closing, but if those exist, they're a
sign of a larger bug elsewhere that should be addressed. On the other
hand, tun registration handles might legitimately be open during driver
upgrades. This also saves us the trouble of dereferencing a freed
FileObject as in the general case.
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
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