LU-12566 mdc: hold lock while walking changelog dev list

LU-12566 mdc: hold lock while walking changelog dev list

In mdc_changelog_cdev_finish() we need chlg_registered_dev_lock
while walking and changing entries on the chlog_registered_devs
and ced_obds lists in chlg_registered_dev_find_by_obd().

Move the calling of chlg_registered_dev_find_by_obd() under the
mutex, and add assertions to the places where the lists are walked
and changed that the mutex is held.

Fixes: 1d40214d96dd ("LU-7659 mdc: expose changelog through char devices")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ib62fdff87cde6a4bcfb9bea24a2ea72a933ebbe5
Reviewed-on: https://review.whamcloud.com/35668
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>