Make sure that highmem pages are not added to symlink page cache

inode_nohighmem() is sufficient to make sure that page_get_link()
won't try to allocate a highmem page.  Moreover, it is sufficient
to make sure that page_symlink/__page_symlink won't do the same
thing.  However, any filesystem that manually preseeds the symlink's
page cache upon symlink(2) needs to make sure that the page it
inserts there won't be a highmem one.

Fortunately, only nfs and shmem have run afoul of that...

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

Documentation/filesystems/porting

@@ -508,7 +508,11 @@ in your dentry operations instead.
 [mandatory]
 	any symlink that might use page_follow_link_light/page_put_link() must
 	have inode_nohighmem(inode) called before anything might start playing with
-	its pagecache.
+	its pagecache.  No highmem pages should end up in the pagecache of such
+	symlinks.  That includes any preseeding that might be done during symlink
+	creation.  __page_symlink() will honour the mapping gfp flags, so once
+	you've done inode_nohighmem() it's safe to use, but if you allocate and
+	insert the page manually, make sure to use the right gfp flags.
 --
 [mandatory]
 	->follow_link() is replaced with ->get_link(); same API, except that