LU-14195 libcfs: switch to kfree_sensitive

LU-14195 libcfs: switch to kfree_sensitive

In Linux 5.10, kzfree() has been renamed kfree_sensitive().

So switch to the new name and provide back-compat support for older
kernels.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: If665168477a0b6241a8ddf31a111cd465fe97783
Reviewed-on: https://review.whamcloud.com/40908
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>