Add missed DMU_PROJECTUSED_OBJECT prefetch.

It seems 9c5167d19f "Project Quota on ZFS" missed to add prefetch
for DMU_PROJECTUSED_OBJECT during scan (scrub/resilver). It should
not cause visible problems, but may affect scub/resilver performance.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15024