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.

Lustre-change: https://review.whamcloud.com/55302
Lustre-commit: 192c395d01062f2e1178ec8ce437f5eea42011c1

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I28563aa60d0f345616fd30cd0899495e7c1ef8f0
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/55506
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>