LU-12511 utils: make kfilnd support a soft requirement

LU-12511 utils: make kfilnd support a soft requirement

The new kfilnd driver doesn't exist upstream and looks like it
will be missing upstream for sometime. Make building the code
for this new LND optional which is needed for the native Linux
Lustre client.

Test-Parameters: trivial
Change-Id: Ib17f78b12ffed95e4198d4524f5ca44aab01c010
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48518
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>