procfs: fix race between symlink removals and traversals

use_pde()/unuse_pde() in ->follow_link()/->put_link() resp.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 6fcdba5..c835b94 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -200,6 +200,7 @@
 	int closing;
 	struct completion *c;
 };
+extern const struct inode_operations proc_link_inode_operations;
 
 extern const struct inode_operations proc_pid_link_inode_operations;