Ensure arc_size_break is 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.

Also use mfu_size and mru_size pulled from the arcstats
kstat file.

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

cmd/arc_summary/arc_summary.py

@@ -185,7 +185,8 @@ def get_arc_summary(Kstat):
 
     # ARC Sizing
     arc_size = Kstat["kstat.zfs.misc.arcstats.size"]
-    mru_size = Kstat["kstat.zfs.misc.arcstats.p"]
+    mru_size = Kstat["kstat.zfs.misc.arcstats.mru_size"]
+    mfu_size = Kstat["kstat.zfs.misc.arcstats.mfu_size"]
     target_max_size = Kstat["kstat.zfs.misc.arcstats.c_max"]
     target_min_size = Kstat["kstat.zfs.misc.arcstats.c_min"]
     target_size = Kstat["kstat.zfs.misc.arcstats.c"]
@@ -231,7 +232,6 @@ def get_arc_summary(Kstat):
 
     output['arc_size_break'] = {}
     if arc_size > target_size:
-        mfu_size = (arc_size - mru_size)
         output['arc_size_break']['recently_used_cache_size'] = {
             'per': fPerc(mru_size, arc_size),
             'num': fBytes(mru_size),
@@ -241,8 +241,7 @@ def get_arc_summary(Kstat):
             'num': fBytes(mfu_size),
         }
 
-    elif arc_size < target_size:
-        mfu_size = (target_size - mru_size)
+    else:
         output['arc_size_break']['recently_used_cache_size'] = {
             'per': fPerc(mru_size, target_size),
             'num': fBytes(mru_size),