media: dvb-core: Fix use-after-free due on race condition at dvb_net

A race condition may occur between the .disconnect function, which
is called when the device is disconnected, and the dvb_device_open()
function, which is called when the device node is open()ed.
This results in several types of UAFs.

The root cause of this is that you use the dvb_device_open() function,
which does not implement a conditional statement
that checks 'dvbnet->exit'.

So, add 'remove_mutex` to protect 'dvbnet->exit' and use
locked_dvb_net_open() function to check 'dvbnet->exit'.

[mchehab: fix a checkpatch warning]

Link: https://lore.kernel.org/linux-media/[email protected]
Signed-off-by: Hyunwoo Kim <[email protected]>
Signed-off-by: Mauro Carvalho Chehab <[email protected]>

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