sheepdog: refine discard support

sheepdog: refine discard support

This patch refines discard support of the sheepdog driver. The
existing discard mechanism was implemented on SD_OP_DISCARD_OBJ, which
was introduced before fine grained reference counting on newer
sheepdog. It doesn't care about relations of snapshots and clones and
discards objects unconditionally.

With this patch, the driver just updates an inode object for updating
reference. Removing the object is done in sheep process side.

Cc: Teruaki Ishizaki <ishizaki.teruaki@lab.ntt.co.jp>
Cc: Vasiliy Tolstov <v.tolstov@selfip.ru>
Cc: Jeff Cody <jcody@redhat.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Tested-by: Vasiliy Tolstov <v.tolstov@selfip.ru>
Message-id: 1441076590-8015-3-git-send-email-mitake.hitoshi@lab.ntt.co.jp
Signed-off-by: Jeff Cody <jcody@redhat.com>