LU-15400 tests: sanity-lfsck MDT_DEVNAME fix

LU-15400 tests: sanity-lfsck MDT_DEVNAME fix

Global MDT_DEVNAME set at the start of sanity-lfsck
equal to a device-mapper device can not be used after
stop() because of a device-mapper device is removed and
facet device is restored:
  stop () ->
     elif dm_flakey_supported $facet; then
        if [[ -n ${!failover_host} && ${!failover_host} != ${!host} ]]
           dm_cleanup_dev $facet ->
              unexport_dm_dev $facet

Without this fix the tests:
    1a, 1b, 1c, 2a, 2b, 2c, 2d, 4, 5, 7a, 7b, 8, 30
fail on failover setup with:
    losetup: /dev/mapper/mds1_flakey: failed to set up loop device

To reproduce the failure just run:
  sh llmountcleanup.sh
  sh sanity-lfsck.sh
on failover setup where mds1_HOST != mds1failover_HOST.

Fixes: 54b9e3f ("LU-684 tests: replace dev_read_only patch with dm-flakey")
Test-Parameters: trivial testlist=sanity-lfsck
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-10667
Reviewed-by: Vladimir Saveliev <vlaidimir.saveliev@hpe.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Change-Id: I2736406161d67335f465cf70eb9f21347a8a798f
Reviewed-on: https://review.whamcloud.com/46025
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>