powerpc/pseries: Fix error return value in cmm_mem_going_offline()

cmm_mem_going_offline() is (only) called from cmm_memory_cb(), which
sends the return value through notifier_from_errno(). The latter
expects 0 or -errno (notifier_to_errno(notifier_from_errno(x)) is 0
for any x >= 0, so passing a positive value cannot make sense). Hence
negate ENOMEM.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/pseries/cmm.c

@@ -574,7 +574,7 @@ static int cmm_mem_going_offline(void *arg)
 				cmm_dbg("Failed to allocate memory for list "
 						"management. Memory hotplug "
 						"failed.\n");
-				return ENOMEM;
+				return -ENOMEM;
 			}
 			memcpy(npa, pa_curr, PAGE_SIZE);
 			if (pa_curr == cmm_page_list)