vfs: protect i_nlink

Prevent direct modification of i_nlink by making it const and adding a
non-const __i_nlink alias.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Tested-by: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

fs/inode.c

diff --git a/fs/inode.c b/fs/inode.c
index ecbb68d..ee4e66b 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -142,7 +142,7 @@
 	atomic_set(&inode->i_count, 1);
 	inode->i_op = &empty_iops;
 	inode->i_fop = &empty_fops;
-	inode->i_nlink = 1;
+	inode->__i_nlink = 1;
 	inode->i_opflags = 0;
 	inode->i_uid = 0;
 	inode->i_gid = 0;

include/linux/fs.h

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 48c1f5f..23467d7 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -768,7 +768,17 @@
 
 	/* Stat data, not accessed from path walking */
 	unsigned long		i_ino;
-	unsigned int		i_nlink;
+	/*
+	 * Filesystems may only read i_nlink directly.  They shall use the
+	 * following functions for modification:
+	 *
+	 *    (set|clear|inc|drop)_nlink
+	 *    inode_(inc|dec)_link_count
+	 */
+	union {
+		const unsigned int i_nlink;
+		unsigned int __i_nlink;
+	};
 	dev_t			i_rdev;
 	loff_t			i_size;
 	struct timespec		i_atime;
@@ -1764,7 +1774,7 @@
  */
 static inline void set_nlink(struct inode *inode, unsigned int nlink)
 {
-	inode->i_nlink = nlink;
+	inode->__i_nlink = nlink;
 }
 
 /**
@@ -1777,7 +1787,7 @@
  */
 static inline void inc_nlink(struct inode *inode)
 {
-	inode->i_nlink++;
+	inode->__i_nlink++;
 }
 
 static inline void inode_inc_link_count(struct inode *inode)
@@ -1799,7 +1809,7 @@
  */
 static inline void drop_nlink(struct inode *inode)
 {
-	inode->i_nlink--;
+	inode->__i_nlink--;
 }
 
 /**
@@ -1812,7 +1822,7 @@
  */
 static inline void clear_nlink(struct inode *inode)
 {
-	inode->i_nlink = 0;
+	inode->__i_nlink = 0;
 }
 
 static inline void inode_dec_link_count(struct inode *inode)