Fix race between proc_readdir and remove_proc_entry

Fix the following race:

proc_readdir				remove_proc_entry
============				=================

spin_lock(&proc_subdir_lock);
[choose PDE to start filldir from]
spin_unlock(&proc_subdir_lock);
					spin_lock(&proc_subdir_lock);
					[find PDE]
					[free PDE, refcount is 0]
					spin_unlock(&proc_subdir_lock);
		    /* boom */
if (filldir(dirent, de->name, ...

[de_put on error path --adobriyan]
Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>