kill f_dentry macro

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

Documentation/filesystems/porting

@@ -467,3 +467,7 @@ in your dentry operations instead.
 [mandatory]
 	d_materialise_unique() is gone; d_splice_alias() does everything you
 	need now.  Remember that they have opposite orders of arguments ;-/
+--
+[mandatory]
+	f_dentry is gone; use f_path.dentry, or, better yet, see if you can avoid
+	it entirely.

include/linux/fs.h

@@ -789,7 +789,6 @@ struct file {
 		struct rcu_head 	fu_rcuhead;
 	} f_u;
 	struct path		f_path;
-#define f_dentry	f_path.dentry
 	struct inode		*f_inode;	/* cached value */
 	const struct file_operations	*f_op;