xfs: fix for variable set but not used warning

Fix below kernel warning:

fs/xfs/scrub/repair.c:539:19: warning: variable 'agno' set but not used [-Wunused-but-set-variable]

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: sunliming <sunliming@kylinos.cn>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>