Ensure arc_size_break are filled in arc_summary.py

When printing output in arc_summary, all possible keys
are expected in the output data structure. For the arc
size breakdown, output is conditionally filled and does
not expect arc target size to be equal to the arc size.

Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>

cmd/arc_summary/arc_summary.py

@@ -241,7 +241,7 @@ def get_arc_summary(Kstat):
             'num': fBytes(mfu_size),
         }
 
-    elif arc_size < target_size:
+    else:
         mfu_size = (target_size - mru_size)
         output['arc_size_break']['recently_used_cache_size'] = {
             'per': fPerc(mru_size, target_size),