ext4: error proc entry creation when the fs/ext4 is not correctly created

From: Shen Feng <shen@cn.fujitsu.com>

When the directory fs/ext4 is not correctly created under proc, the entry
under this directory should not be created.

Signed-off-by: Shen Feng <shen@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index d2c61eb..3582202 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2789,6 +2789,10 @@ static int ext4_mb_init_per_dev_proc(struct super_block *sb)
 	struct proc_dir_entry *proc;
 	char devname[64];
 
+	if (proc_root_ext4 == NULL) {
+		sbi->s_mb_proc = NULL;
+		return -EINVAL;
+	}
 	bdevname(sb->s_bdev, devname);
 	sbi->s_mb_proc = proc_mkdir(devname, proc_root_ext4);