fs: allocate structure unconditionally in seq_open()

Since patch described below, from v2.6.15-rc1, seq_open() could use a
struct seq_file already allocated by the caller if the pointer to the
structure is stored in file->private_data before calling the function.

    Commit 1abe77b0fc
    Author: Al Viro <viro@zeniv.linux.org.uk>
    Date:   Mon Nov 7 17:15:34 2005 -0500

        [PATCH] allow callers of seq_open do allocation themselves

        Allow caller of seq_open() to kmalloc() seq_file + whatever else they
        want and set ->private_data to it.  seq_open() will then abstain from
        doing allocation itself.

As there's no more use for such feature, as it could be easily replaced by
calls to seq_open_private() (see commit 39699037a5 ("[FS] seq_file:
Introduce the seq_open_private()")) and seq_release_private() (see
v2.6.0-test3), support for this uncommon feature can be removed from
seq_open().

Link: http://lkml.kernel.org/r/cover.1433193673.git.ydroneaud@opteya.com
Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/seq_file.c

@@ -51,15 +51,16 @@ static void *seq_buf_alloc(unsigned long size)
  */
 int seq_open(struct file *file, const struct seq_operations *op)
 {
-	struct seq_file *p = file->private_data;
+	struct seq_file *p;
+
+	WARN_ON(file->private_data);
+
+	p = kzalloc(sizeof(*p), GFP_KERNEL);
+	if (!p)
+		return -ENOMEM;
+
+	file->private_data = p;
 
-	if (!p) {
-		p = kmalloc(sizeof(*p), GFP_KERNEL);
-		if (!p)
-			return -ENOMEM;
-		file->private_data = p;
-	}
-	memset(p, 0, sizeof(*p));
 	mutex_init(&p->lock);
 	p->op = op;
 #ifdef CONFIG_USER_NS