xfs: fix uninitialized variable in xfs_scrub_quota

On the first pass through the while(1) loop, we get to
xfs_scrub_should_terminate() which can test the uninitialized
error variable.

Fixes-coverity-id: 1423737
Fixes: c2fc338c ("xfs: scrub quota information")
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>