viotape: Use unlocked_ioctl

This pushes the BKL down into the driver.  Based on a patch by Alan Cox.

We need to do it this way for now as the inode parameter of viotap_ioctl
is used internally as a flag.  We should do a further cleanup patch.

Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>