LU-13562 build: get correct kernel flavor for SLES

LU-13562 build: get correct kernel flavor for SLES

This patch fixes lustre.spec.in to get correct kernel flavor
for SLES when kobjdir is detected as /lib/modules/%{_kver}/build.

Test-Parameters: trivial clientdistro=sles15sp3

Change-Id: I350032af383ea8b7f48accd93e5cd11c571e6620
Fixes: d746e64fe1 ("LU-13562 build: SUSE build support for azure, cray_ari_s")
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/47596
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>