mem: cdev lock_kernel() pushdown

It's really hard to tell if this is necessary - lots of weird
magic happens by way of map_devmem()

Signed-off-by: Jonathan Corbet <[e-mail]>

drivers/char/mem.c

···
 #include <linux/bootmem.h>
 #include <linux/splice.h>
 #include <linux/pfn.h>
+#include <linux/smp_lock.h>
 
 #include <asm/uaccess.h>
 #include <asm/io.h>
···
 
 static int memory_open(struct inode * inode, struct file * filp)
 {
+	int ret = 0;
+
+	lock_kernel();
 	switch (iminor(inode)) {
 		case 1:
 			filp->f_op = &mem_fops;
···
 			break;
 #endif
 		default:
+			unlock_kernel();
 			return -ENXIO;
 	}
 	if (filp->f_op && filp->f_op->open)
-		return filp->f_op->open(inode,filp);
-	return 0;
+		ret = filp->f_op->open(inode,filp);
+	unlock_kernel();
+	return ret;
 }
 
 static const struct file_operations memory_fops = {