LU-18687 compat: move generic-radix-tree to lustre_compat

LU-18687 compat: move generic-radix-tree to lustre_compat

Migrate the backported radix tree code to lustre_compat.

Eventually, all of the Lustre/LNet compatability code
will live in lustre_compat - maintaining a clear
separation from the functional code in Lustre and LNet.

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: Iaf6fed877b23829be948f1347d21e1ff7b9ce5a9
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58148
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>