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.

Lustre-change: https://review.whamcloud.com/47594
Lustre-commit: 0a2d42c7efc5378286d0e0b4255c3e108464fd47

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/47754
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>