LU-6891 test: fix grow_xattr() defect

LU-6891 test: fix grow_xattr() defect

string needs to be quoted to avoid syntax error on
DNE setup

Signed-off-by: Elena Gryaznova <elena.gryaznova@seagate.com>
Seagate-bug-id: MRP-2762
Reviewed-by: Andriy Skulysh <andriy.skulysh@seagate.com>
Change-Id: Ibdb383f2581fbe4bd9e92f9d89a98bfe14209f23
Reviewed-on: http://review.whamcloud.com/15672
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>