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 --cc fs/proc/generic.c
Simple merge

diff --cc fs/proc/inode.c
Simple merge

diff --cc fs/proc/internal.h
Simple merge