LU-12030 tests: Properly detect debug kernel use on rhel7.6

LU-12030 tests: Properly detect debug kernel use on rhel7.6

kmalloc-128 slab seems to be gone, so let's use dma-kmalloc-128
instead.

Lustre-commit: 673e8b0b3c404b14a02e7e874c3aed991bc52eb3
Lustre-change: https://review.whamcloud.com/34342

Test-Parameters: trivial testlist=conf-sanity env=ONLY=63
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Change-Id: Ice7f350ba2bc6cc733c0a98b0037e6f0980216c9
Reviewed-on: https://review.whamcloud.com/38572
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>