LU-16745 mdc: md_open_data should keep ref on close_req

LU-16745 mdc: md_open_data should keep ref on close_req

md_open_data should keep a ref on mod_close_req,
otherwise the mod_close_req could be freed before
we try to access mod_close_req via md_open_data.

Lustre-change: https://review.whamcloud.com/50656
Lustre-commit: ca716f763f89af192ab86678ee9d14f49c80cae6

Change-Id: I621f7db389854326db298d99957a0bce43024b6e
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51198
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>