nbd: add locking to nbd_ioctl

nbd: add locking to nbd_ioctl

The code was written to rely on big kernel lock to protect it from races.
It mostly works when interface is not abused.

So this uses tx_lock to protect data structures from concurrent use
between ioctl and worker threads.

Next step will be moving from ioctl to unlocked_ioctl.

[[email protected]: coding-style fixes]
[[email protected]: add missing return]
Signed-off-by: Pavel Machek <[email protected]>
Acked-by: Paul Clements <[email protected]>
Cc: Jens Axboe <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>