Use append mode in 'dd' when the desired swap size is not a multiple of 64M (#2286)

First 'dd' command creates the swap in multiples of 64M and the second
is supposed to add the rest, however, with no 'oflag=append' it just
overwrites what was already written so the file doesn't grow as desired.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>

Author: vittyvk

azurelinuxagent/daemon/resourcedisk/default.py

@@ -373,6 +373,7 @@ def mkfile(self, filename, nbytes):
         if blocks > 0:
             ret = shellutil.run(dd_cmd.format(dd_maxbs, blocks, fn_sh)) << 8
 
+        dd_cmd += " oflag=append"
         remains = int(nbytes % dd_maxbs)
         if remains > 0:
             ret += shellutil.run(dd_cmd.format(remains, 1, fn_sh))