LU-15103 tests: clean up busy cifs mount

LU-15103 tests: clean up busy cifs mount

Busy cifs mount point makes cleanup_cifs fail which
will infact fails lustre unmount as cifs mount point is
lustre mount.

Test-Parameters: trivial
Signed-off-by: gaurav mahajan <gaurav.mahajan@seagate.com>
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-4146
Reviewed-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Change-Id: I4b7ec7e0a6a706198e328dad337648bf3cb2c3be
Reviewed-on: https://review.whamcloud.com/45238
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>