fs: remove vfs_follow_link

For a long time no filesystem has been using vfs_follow_link, and as seen
by recent filesystem submissions any new use is accidental as well.

Remove vfs_follow_link, document the replacement in
Documentation/filesystems/porting and also rename __vfs_follow_link
to match its only caller better.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 206a1bd..f089058 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -451,3 +451,7 @@
 --
 [mandatory]
 	->readdir() is gone now; switch to ->iterate()
+[mandatory]
+	vfs_follow_link has been removed.  Filesystems must use nd_set_link
+	from ->follow_link for normal symlinks, or nd_jump_link for magic
+	/proc/<pid> style links.