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.

Lustre-change: https://review.whamcloud.com/50937
Lustre-commit: 0e9708016b9948676484d290326c1fe8a269eb80
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/+/55117
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>