LU-14632 tests: fix sanity-hsm test_606()

LU-14632 tests: fix sanity-hsm test_606()

After check_and_setup_lustre() mds1_dev is equal to
/dev/mapper/mds1_flakey, which is unexported to saved real
device  (/dev/vdc) by stop():
  stop mds1
    elif dm_flakey_supported mds1; then
        dm_cleanup_dev mds1
          unexport_dm_dev mds1
As a result stack_trap() is called with non existing
/dev/mapper/mds1_flakey:
  stack_trap 'stop mds1; start mds1 /dev/mapper/mds1_flakey
              -o rw,user_xattr' EXIT
and failed as:
  losetup: /dev/mapper/mds1_flakey: failed to set up loop device:
              No such file or directory
Reproducer:
  run ONLY=606 sh sanity-hsm.sh on "failover" setup (mds1_HOST !=
  mds1failover_HOST), no llmount.sh before the test

Fixes: 54b9e3f78935 ("LU-684 tests: replace dev_read_only patch with dm-flakey")
Test-Parameters: trivial testlist=sanity-hsm env=ONLY=606
Test-Parameters: fstype=zfs testlist=sanity-hsm env=ONLY=606
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-9920
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Andriy Skulysh <andriy.skulysh@hpe.com>
Change-Id: I9ab3cbcf67c6fd046861810a2ceab262f211436b
Reviewed-on: https://review.whamcloud.com/43409
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>