LU-13745 tests: skip sanity test_426 for 4.18+

LU-13745 tests: skip sanity test_426 for 4.18+

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.

 Unable to handle NULL pointer dereference at address 0000000000000004
 user pgtable: 64k pages, 48-bit VAs, pgdp = 000000009f14b2d0
 Internal error: Oops: 96000005 [#1] SMP
 CPU: 1 PID: 11273 Comm: ptlrpcd_01_01 4.18.0-147.8.1.el8_1.aarch64 #1
 Process ptlrpcd_01_01 (pid: 11273, stack limit = 0x00000000f9135a93)
 Call trace:
  mempool_free+0x24/0xe0
  llcrypt_free_bounce_page.part.1+0x38/0x48 [libcfs]
  llcrypt_free_bounce_page+0x24/0x30 [libcfs]
  brw_interpret+0x124/0x10c8 [osc]
  ptlrpc_check_set+0x688/0x3318 [ptlrpc]
  ptlrpcd_check+0x470/0x820 [ptlrpc]
  ptlrpcd+0x3d4/0x5c8 [ptlrpc]
  kthread+0x130/0x138

Test-Parameters: trivial clientdistro=el8.1 clientarch=aarch64
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I8f7b1d5e3ee69a3e0a6dfe3944949741a74cb62a
Reviewed-on: https://review.whamcloud.com/40326
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>