phonedev: cdev lock_kernel() pushdown

phone_open() looks OK, but I don't trust the subsidiary drivers (and ixj in
particular).
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

drivers/telephony/phonedev.c

@@ -23,6 +23,7 @@
 #include <linux/errno.h>
 #include <linux/phonedev.h>
 #include <linux/init.h>
+#include <linux/smp_lock.h>
 #include <asm/uaccess.h>
 #include <asm/system.h>
 
@@ -53,6 +54,7 @@ static int phone_open(struct inode *inode, struct file *file)
 	if (minor >= PHONE_NUM_DEVICES)
 		return -ENODEV;
 
+	lock_kernel();
 	mutex_lock(&phone_lock);
 	p = phone_device[minor];
 	if (p)
@@ -79,6 +81,7 @@ static int phone_open(struct inode *inode, struct file *file)
 	fops_put(old_fops);
 end:
 	mutex_unlock(&phone_lock);
+	unlock_kernel();
 	return err;
 }