LU-8130 libcfs: don't use radix tree for xarray

LU-8130 libcfs: don't use radix tree for xarray

For newer kernels the radix tree is totally based on Xarray. For Lustre
support for RHEL7 we backported Xarray but it still was using the
radix tree. Their is a mismatch between what the radix tree expects
and using a struct xa_node when allocating and freeing memory. Instead
abandon all use of the radix tree with Xarray. We use our own private
kmem cache which is based on radix tree but it uses xa_node.

Test-Parameters: trivial
Fixes: 84e12028be9a ("LU-9859 libcfs: add support for Xarray")
Change-Id: I87607aa0e55a4aca039f2fef5a76fbff0bedd9b3
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51840
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>