sheep returns an invalid parameter error when receiving a FLUSH_VDI operation

[Yuichi Bando]

When receiving a FLUSH_VDI operation,
sheep returns an SD_RES_INVALID_PARMS instead of SUCCESS if object cache option is off.
And that time, sheep outputs a large amount of below log.

=== log ===
Feb 05 20:12:01 DEBUG [io 7198] do_process_work(1392) failed: 16, 809ff93000000000 , 1, Invalid parameters

=== FLUSH VDI operation ===
[SD_OP_FLUSH_VDI] = {
  .name = "FLUSH_VDI",
  .type = SD_OP_TYPE_LOCAL,
  .process_work = local_flush_vdi,
:
:

This problem does not affect the normal sheepdog operation,
but we should modify it.

Thanks
Yuichi Bando