xfs: fix scrub trace with null pointer in quotacheck

The quotacheck doesn't initialize sc->ip.

Cc: stable@vger.kernel.org # v6.8
Fixes: 21d75009 ("xfs: improve dquot iteration for scrub")
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>