powerpc: Use WARN instead of dump_stack when printing EEH error backtrace

When we get an EEH error we just print a backtrace with dump_stack
which is rather cryptic. We really should print something before
spewing out the backtrace.

Also switch from dump_stack to WARN so we get more information about
the fail - what modules were loaded, what process was running etc.
This was useful information when debugging a recent EEH subsystem bug.

The standard WARN output should also get picked up by monitoring
tools like kerneloops.

The register dump is of questionable value here but I figured it was
better to use something standard and not roll my own.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/eeh.c

@@ -489,7 +489,7 @@ int eeh_dn_check_failure(struct device_node *dn, struct pci_dev *dev)
 	 * a stack trace will help the device-driver authors figure
 	 * out what happened.  So print that out.
 	 */
-	dump_stack();
+	WARN(1, "EEH: failure detected\n");
 	return 1;
 
 dn_unlock: