LU-13745 tests: skip sanity test_426 for 4.15+

LU-13745 tests: skip sanity test_426 for 4.15+

Add sanity test_426 to ALWAYS_EXCEPT for newer client kernels because
it is crashing 100% since "LU-13745 test: add splice test for lustre"
was landed.

Test-Parameters: trivial clientdistro=ubuntu1804
Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I84a722a27c3e8a572c20b46ca9daaf44e8720b54
Reviewed-on: https://review.whamcloud.com/40366
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>