powerpc/pseries: Don't attempt to acquire drc during memory hot add for assigned lmbs

Check if an LMB is assigned before attempting to call dlpar_acquire_drc
in order to avoid any unnecessary rtas calls. This substantially
reduces the running time of memory hot add on lpars with large amounts
of memory.

[mpe: We need to explicitly set rc to 0 in the success case, otherwise
 the compiler might think we use rc without initialising it.]

Fixes: c21f515c7436 ("powerpc/pseries: Make the acquire/release of the drc for memory a seperate step")
Cc: stable@vger.kernel.org # v4.11+
Signed-off-by: John Allen <jallen@linux.vnet.ibm.com>
Reviewed-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/pseries/hotplug-memory.c

@@ -828,6 +828,9 @@ static int dlpar_memory_add_by_count(u32 lmbs_to_add, struct property *prop)
 		return -EINVAL;
 
 	for (i = 0; i < num_lmbs && lmbs_to_add != lmbs_added; i++) {
+		if (lmbs[i].flags & DRCONF_MEM_ASSIGNED)
+			continue;
+
 		rc = dlpar_acquire_drc(lmbs[i].drc_index);
 		if (rc)
 			continue;
@@ -870,6 +873,7 @@ static int dlpar_memory_add_by_count(u32 lmbs_to_add, struct property *prop)
 				lmbs[i].base_addr, lmbs[i].drc_index);
 			lmbs[i].reserved = 0;
 		}
+		rc = 0;
 	}
 
 	return rc;