dio: add partition support

README

@@ -167,7 +167,8 @@ vicious.widgets.fs
 
 vicious.widgets.dio
   - provides I/O statistics for requested storage devices
-  - takes the disk as an argument, i.e. "sda"
+  - takes the disk as an argument, i.e. "sda" (or specific partition,
+    i.e. "sda/sda5")
   - returns a table with string keys: {total_s}, {total_kb}, {total_mb},
     {read_s}, {read_kb}, {read_mb}, {write_s}, {write_kb}, {write_mb}
     and {sched}

widgets/dio.lua

@@ -61,7 +61,7 @@ local function worker(format, disk)
     helpers.uformat(disk_usage[disk], "total", diff_total[disk][7] + diff_total[disk][3], unit)
 
     -- Store I/O scheduler
-    if disk_stats.queue.scheduler then
+    if disk_stats.queue and disk_stats.queue.scheduler then
         disk_usage[disk]["{sched}"] = string.gmatch(disk_stats.queue.scheduler, "%[([%a]+)%]")
     end