LU-9859 libcfs: prepare for switch to container_of_safe()

LU-9859 libcfs: prepare for switch to container_of_safe()

Upstream Linux uses container_of_safe() rather than
container_of0().

So provide container_of_safe() for all kernels, and use spelling.txt
to suggest new code uses it.  We can then start switching code
over gradually.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ie938b76d569de6bb10b51fb0b12d79fcd0f43e7e
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Reviewed-on: https://review.whamcloud.com/37736
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>