LU-13783 libcfs: provide fallback kallsyms_lookup_name()

LU-13783 libcfs: provide fallback kallsyms_lookup_name()

Since Linux 5.7, kallsyms_lookup_name() is no longer exported, so we
cannot rely on it.

So test for this, and when not available provide a fallback which just
returns NULL.

As this was the only way to access apply_workqueue_attrs() in recent
kernels, we need to cope with the absence of that function.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I09cc00047ec163a9395c5acd415505a8586e4e99
Reviewed-on: https://review.whamcloud.com/40826
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>