fix issue jthornber#69 - ENOENT /sys/block/<dev>/queue/discard_granul…

…arity

A lot of tests fail due to Errno::ENOENT: No such file or directory @ rb_sysopen - /sys/block/<dev>/queue/discard_granularity when dev is a partition instead of a disk, so we should first find the disk of the partition and then read the discard_granularity file of the disk.

lib/dmtest/discard_limits.rb

@@ -13,8 +13,13 @@ def initialize(dev)
   private
 
   def read_param(p)
+    disk = dev.to_s
+    type = `lsblk -n /dev/#{disk} | grep -w #{disk} | sed 's/[ \t]*$//; s/.*[ \t]//'`.strip
+    if type == "part"
+      disk = `basename $(dirname $(find /sys -type d -name #{disk}))`.strip
+    end
     line = ''
-    File.open("/sys/block/#{dev.to_s}/queue/discard_#{p}", 'r') do |file|
+    File.open("/sys/block/#{disk}/queue/discard_#{p}", 'r') do |file|
       line = file.gets
     end