proc 1/2: do PDE usecounting even for ->read_proc, ->write_proc

struct proc_dir_entry::owner is going to be removed. Now it's only necessary
to protect PDEs which are using ->read_proc, ->write_proc hooks.

However, ->owner assignments are racy and make it very easy for someone to switch
->owner on live PDE (as some subsystems do) without fixing refcounts and so on.

http://bugzilla.kernel.org/show_bug.cgi?id=12454

So, ->owner is on death row.

Proxy file operations exist already (proc_file_operations), just bump usecount
when necessary.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index d8bb5c6..e11dc22 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -127,7 +127,7 @@
 		complete(pde->pde_unload_completion);
 }
 
-static void pde_users_dec(struct proc_dir_entry *pde)
+void pde_users_dec(struct proc_dir_entry *pde)
 {
 	spin_lock(&pde->pde_unload_lock);
 	__pde_users_dec(pde);