LU-11418 mdd: delete name if orphan doesn't exist

LU-11418 mdd: delete name if orphan doesn't exist

mdd_orphan_destroy() should delete name if orphan object doesn't
exist, otherwise the orphan clean thread will try to destroy this
orphan in dead loop.

add sanity test_811.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Id22b2fab0ac87dfb81ca9f01d8ed0338f1b12120
Reviewed-on: https://review.whamcloud.com/33661
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>