[SCSI] ch: Convert to use unlocked_ioctl

As of now, compat_ioctl already runs without the BKL, whereas ioctl runs
with the BKL. This patch first converts changer_fops to use a .unlocked_ioctl
member. It applies the same locking rationale than ch_ioctl_compat() uses
to ch_ioctl().

Signed-off-by: Mathieu Segaud <mathieu.segaud@regala.cx>
Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>