LU-17897 lfsck: don't assert on orphan existence

LU-17897 lfsck: don't assert on orphan existence

lfsck_namespace_create_orphan_dir() is called in several cases,
and orphan may exist in some cases, change assertion to check.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I28563aa60d0f345616fd30cd0899495e7c1ef8f0
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55302
Tested-by: jenkins <devops@whamcloud.com>
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>