LU-10204 tests: Fix line continuation

LU-10204 tests: Fix line continuation

There is a simple  missing in one of the error paths in
ladvise_willread_performance which results in an unknown command
error. Fix it by adding the missing backslash.

Test-Parameters: trivial
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Change-Id: Ifff854c6cf5c1add71fd1571cb4de01d56204fde
Reviewed-on: https://review.whamcloud.com/29981
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>