LU-14441 mdc: check/grab import before access

LU-14441 mdc: check/grab import before access

to ensure the import doesn't disappear while being accessed
via procfs.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I005c96b349e55646996fd0d265ab4dd1e2b9a1fa
Reviewed-on: https://review.whamcloud.com/41681
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>