dnode: allow storage class to be overridden by object type

spa_preferred_class() selects a storage class based on (among other
things) the DMU object type. This only works for old-style object types
that match only one specific kind of thing. For DMU_OTN_ types we need
another way to signal the storage class.

This commit allows the object type to be overridden in the IO policy for
the purposes of choosing a storage class. It then adds the ability to
set the storage type on a dnode hold, such that all writes generated
under that hold will get it.

This method has two shortcomings:

- it would be better if we could "name" a set of storage class
  preferences rather than it being implied by the object type.
- it would be better if this info were stored in the dnode on disk.

In the absence of those things, this seems like the smallest possible
change.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: iXsystems, Inc.
Closes #15894

module/zfs/dmu.c

@@ -2362,6 +2362,7 @@ dmu_write_policy(objset_t *os, dnode_t *dn, int level, int wp, zio_prop_t *zp)
 	memset(zp->zp_mac, 0, ZIO_DATA_MAC_LEN);
 	zp->zp_zpl_smallblk = DMU_OT_IS_FILE(zp->zp_type) ?
 	    os->os_zpl_special_smallblock : 0;
+	zp->zp_storage_type = dn ? dn->dn_storage_type : DMU_OT_NONE;
 
 	ASSERT3U(zp->zp_compress, !=, ZIO_COMPRESS_INHERIT);
 }