Add a missing unlock_kernel() in raw_open()

upstream commit: 996ff68d

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>