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.

Change-Id: I621f7db389854326db298d99957a0bce43024b6e
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50656
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>