net: Remove BKL from tun

The lock_kernel/unlock_kernel() in cycle_kernel_lock() which is called
in tun_chr_open() is not serializing against anything and safe to
remove.

tun_chr_fasync() is serialized by get/put_tun() and fasync_helper()
has no dependency on BKL. The modification of tun->flags is racy with
and without the BKL so removing it does not make it worse.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>