x86, msr: fix NULL pointer deref due to msr_open on nonexistent CPUs

msr_open tests for someone trying to open a device for a nonexistent CPU.
However, the function always returns 0, not ret like it should, hence
userspace can BUG the kernel trivially.  This bug was introduced by the
cdev lock_kernel pushdown patch last May.

The BUG can be reproduced with these commands:

# mknod fubar c 202 8 <-- pick a number less than NR_CPUS that is not
                          the number of an online CPU
# cat fubar
Signed-off-by: NDarrick J. Wong <djwong@us.ibm.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/msr.c

@@ -131,7 +131,7 @@ static int msr_open(struct inode *inode, struct file *file)
 		ret = -EIO;	/* MSR not supported */
 out:
 	unlock_kernel();
-	return 0;
+	return ret;
 }
 
 /*