constify alloc_file()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/file.h b/include/linux/file.h
index 7444f5f..61eb82c 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -17,7 +17,7 @@ struct file_operations;
 struct vfsmount;
 struct dentry;
 struct path;
-extern struct file *alloc_file(struct path *, fmode_t mode,
+extern struct file *alloc_file(const struct path *, fmode_t mode,
 	const struct file_operations *fop);
 
 static inline void fput_light(struct file *file, int fput_needed)