LU-15220 libcfs: fix panic_notifier_list undeclared error

LU-15220 libcfs: fix panic_notifier_list undeclared error

In kernel 5.14 commit f39650de687e35766572ac89dbcd16a5911e2f0a,
panic and oops helpers are split out from include/linux/kernel.h.

This patch accommodates the above changes and fixes the
"'panic_notifier_list' undeclared" error.

Change-Id: I6888f9f4878906c572bb40d950a70ff642d3474e
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/45812
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>