LU-13783 libcfs: don't lookup symbol when not needed

LU-13783 libcfs: don't lookup symbol when not needed

Linux 5.8 and later do not export kallsyms_lookup_name() so we need to
remove dependence on it.
Looking up radix_tree_node_cachep is not needed in these kernels,
or any kernel were HAVE_XARRAY_SUPPORT is defined.
So avoid the lookup in those cases.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ia61fe9a025653985b7dcf67b1234d32667719956
Reviewed-on: https://review.whamcloud.com/40825
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>