LU-16819 build: use mofed path based on target kernel

LU-16819 build: use mofed path based on target kernel

Instead of using "uname -r", which limits builds to the currently
running kernel, use the target kernel which is available in
LINUXRELEASE, if the directory is available.
Building for a specific kernel is common practice when using DKMS.

Test-Parameters: trivial
Signed-off-by: Ake Sandgren <ake.sandgren@hpc2n.umu.se>
Change-Id: Ifce912061a74fc5b7435cd940105190f0c3cd544
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50937
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>