proc: less special case in xlate code

If valid "parent" is passed to proc_create/remove_proc_entry(), then name of
PDE should consist of only one path component, otherwise creation or or
removal will fail.  However, if NULL is passed as parent then create/remove
accept full path as a argument.  This is arbitrary restriction -- all
infrastructure is in place.

So, patch allows the following to succeed:

	create_proc_entry("foo/bar", 0, pde_baz);
	remove_proc_entry("baz/foo/bar", &proc_root);

Also makes the following to behave identically:

	create_proc_entry("foo/bar", 0, NULL);
	create_proc_entry("foo/bar", 0, &proc_root);

Discrepancy noticed by Den Lunev (IIRC).

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>