proc: more debugging for "already registered" case

Print parent directory name as well.

The aim is to catch non-creation of parent directory when proc_mkdir will
return NULL and all subsequent registrations go directly in /proc instead
of intended directory.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[ Fixed insane printk string while at it.  - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>