Ensure that gang_copies is always at least as large as copies

As discussed in the comments of PR #17004, you can theoretically run
into a case where a gang child has more copies than the gang header,
which can lead to some odd accounting behavior (and even trip a
VERIFY). While the accounting code could be changed to handle this, it
fundamentally doesn't seem to make a lot of sense to allow this to
happen. If the data is supposed to have a certain level of reliability,
that isn't actually achieved unless the gang_copies property is set to
match it.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Alexander Motin <[email protected]>
Signed-off-by: Paul Dagnelie <[email protected]>
Closes #17484

Author: pcd1193182

module/zfs/dmu.c

@@ -2500,7 +2500,8 @@ dmu_write_policy(objset_t *os, dnode_t *dn, int level, int wp, zio_prop_t *zp)
 	zp->zp_type = (wp & WP_SPILL) ? dn->dn_bonustype : type;
 	zp->zp_level = level;
 	zp->zp_copies = MIN(copies, spa_max_replication(os->os_spa));
-	zp->zp_gang_copies = MIN(gang_copies, spa_max_replication(os->os_spa));
+	zp->zp_gang_copies = MIN(MAX(gang_copies, copies),
+	    spa_max_replication(os->os_spa));
 	zp->zp_dedup = dedup;
 	zp->zp_dedup_verify = dedup && dedup_verify;
 	zp->zp_nopwrite = nopwrite;