constify file_inode()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -1874,7 +1874,7 @@ extern int current_umask(void);
 extern void ihold(struct inode * inode);
 extern void iput(struct inode *);
 
-static inline struct inode *file_inode(struct file *f)
+static inline struct inode *file_inode(const struct file *f)
 {
 	return f->f_inode;
 }