Add missed DMU_PROJECTUSED_OBJECT prefetch.

It seems 9c5167d "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

module/zfs/dsl_scan.c

@@ -2015,6 +2015,11 @@ dsl_scan_prefetch_cb(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
 		    zb->zb_objset, DMU_META_DNODE_OBJECT);
 
 		if (OBJSET_BUF_HAS_USERUSED(buf)) {
+			if (OBJSET_BUF_HAS_PROJECTUSED(buf)) {
+				dsl_scan_prefetch_dnode(scn,
+				    &osp->os_projectused_dnode, zb->zb_objset,
+				    DMU_PROJECTUSED_OBJECT);
+			}
 			dsl_scan_prefetch_dnode(scn,
 			    &osp->os_groupused_dnode, zb->zb_objset,
 			    DMU_GROUPUSED_OBJECT);