fs: namespace: suppress 'may be used uninitialized' warnings

The gcc version 4.9.1 compiler complains Even though it isn't possible for
these variables to not get initialized before they are used.

fs/namespace.c: In function ‘SyS_mount’:
fs/namespace.c:2720:8: warning: ‘kernel_dev’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  ret = do_mount(kernel_dev, kernel_dir->name, kernel_type, flags,
        ^
fs/namespace.c:2699:8: note: ‘kernel_dev’ was declared here
  char *kernel_dev;
        ^
fs/namespace.c:2720:8: warning: ‘kernel_type’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  ret = do_mount(kernel_dev, kernel_dir->name, kernel_type, flags,
        ^
fs/namespace.c:2697:8: note: ‘kernel_type’ was declared here
  char *kernel_type;
        ^

Fix the warnings by simplifying copy_mount_string() as suggested by Al Viro.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/compat.c

@@ -797,8 +797,9 @@ COMPAT_SYSCALL_DEFINE5(mount, const char __user *, dev_name,
 	struct filename *dir;
 	int retval;
 
-	retval = copy_mount_string(type, &kernel_type);
-	if (retval < 0)
+	kernel_type = copy_mount_string(type);
+	retval = PTR_ERR(kernel_type);
+	if (IS_ERR(kernel_type))
 		goto out;
 
 	dir = getname(dir_name);
@@ -806,8 +807,9 @@ COMPAT_SYSCALL_DEFINE5(mount, const char __user *, dev_name,
 	if (IS_ERR(dir))
 		goto out1;
 
-	retval = copy_mount_string(dev_name, &kernel_dev);
-	if (retval < 0)
+	kernel_dev = copy_mount_string(dev_name);
+	retval = PTR_ERR(kernel_dev);
+	if (IS_ERR(kernel_dev))
 		goto out2;
 
 	retval = copy_mount_options(data, &data_page);

fs/internal.h

@@ -51,7 +51,7 @@ extern int vfs_path_lookup(struct dentry *, struct vfsmount *,
  * namespace.c
  */
 extern int copy_mount_options(const void __user *, unsigned long *);
-extern int copy_mount_string(const void __user *, char **);
+extern char *copy_mount_string(const void __user *);
 
 extern struct vfsmount *lookup_mnt(struct path *);
 extern int finish_automount(struct vfsmount *, struct path *);

fs/namespace.c

@@ -2514,21 +2514,9 @@ int copy_mount_options(const void __user * data, unsigned long *where)
 	return 0;
 }
 
-int copy_mount_string(const void __user *data, char **where)
+char *copy_mount_string(const void __user *data)
 {
-	char *tmp;
-
-	if (!data) {
-		*where = NULL;
-		return 0;
-	}
-
-	tmp = strndup_user(data, PAGE_SIZE);
-	if (IS_ERR(tmp))
-		return PTR_ERR(tmp);
-
-	*where = tmp;
-	return 0;
+	return data ? strndup_user(data, PAGE_SIZE) : NULL;
 }
 
 /*
@@ -2794,8 +2782,9 @@ SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
 	char *kernel_dev;
 	unsigned long data_page;
 
-	ret = copy_mount_string(type, &kernel_type);
-	if (ret < 0)
+	kernel_type = copy_mount_string(type);
+	ret = PTR_ERR(kernel_type);
+	if (IS_ERR(kernel_type))
 		goto out_type;
 
 	kernel_dir = getname(dir_name);
@@ -2804,8 +2793,9 @@ SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
 		goto out_dir;
 	}
 
-	ret = copy_mount_string(dev_name, &kernel_dev);
-	if (ret < 0)
+	kernel_dev = copy_mount_string(dev_name);
+	ret = PTR_ERR(kernel_dev);
+	if (IS_ERR(kernel_dev))
 		goto out_dev;
 
 	ret = copy_mount_options(data, &data_page);