LU-11617 mdc: fix possible deadlock in chlg_open()

LU-11617 mdc: fix possible deadlock in chlg_open()

Lockdep reports a possible deadlock between chlg_open() and
mdc_changelog_cdev_init()

mdc_changelog_cdev_init() takes chlg_registered_dev_lock and then
calls misc_register() which takes misc_mtx.
chlg_open() is called while misc_mtx is held, and tries to take
chlg_registered_dev_lock.
If these two functions race, a deadlock can occur as each thread will
hold one of the locks while trying to take the other.

chlg_open() does not need to take a lock.  It only uses the
lock to stablize a list while looking for the matching
chlg_registered_dev, and this can be found directly by examining
file->private_data.

So remove chlg_obd_get(), and use file->private_data to find the
obd_device.
Also ensure the device is fully initialized before calling
misc_register().  This means setting up some list linkage before the
call, and tearing it down if there is an error.

Lustre-change: https://review.whamcloud.com/33572
Lustre-commit: 206b21741b07a10269bbcfdac28743591b64ab2f

Change-Id: Icffdebcee656ee6199297ba2a28ba57dcbc51ae1
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36230
Reviewed-by: Neil Brown <neilb@suse.de>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>