powerpc/pseries: Correct string length in pseries_of_derive_parent()

Commit a030e1e4 make a change to use
kstrndup() instead of kmalloc() + strlcpy() in the pseries_of_derive_parent()
routine that introduces a subtle change in the parent path name generated.
The kstrndup() routine will copy n characters followed by a terminating null,
whereas strlcpy() will copy n-1 characters and add a terminating null.

This slight difference results in having a parent path that includes the
tailing '/' character, "/cpus/" vs. "/cpus". This then causes the subsequent
call to of_find_node_by_path() to fail, and in the case of DLPAR add
operations the DLPAR request fails.

This patch decrements the pointer returned from kbasename() to point to the
'/' character before the base name instead of the base name. This then
adjusts the string length calculations to not include the trailing '/'
in the parent path name.
Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/pseries/of_helpers.c

@@ -17,13 +17,16 @@ struct device_node *pseries_of_derive_parent(const char *path)
 {
 	struct device_node *parent;
 	char *parent_path = "/";
-	const char *tail = kbasename(path);
+	const char *tail;
+
+	/* We do not want the trailing '/' character */
+	tail = kbasename(path) - 1;
 
 	/* reject if path is "/" */
 	if (!strcmp(path, "/"))
 		return ERR_PTR(-EINVAL);
 
-	if (tail > path + 1) {
+	if (tail > path) {
 		parent_path = kstrndup(path, tail - path, GFP_KERNEL);
 		if (!parent_path)
 			return ERR_PTR(-ENOMEM);