Fix dnode byteswapping

If a dnode has a spill pointer, and we use DN_SLOTS_TO_BONUSLEN() then
we will possibly include the spill pointer in the len calculation and it
will be byteswapped. Then dnode_byteswap() will carry on and swap the
spill pointer again. Fix this by using DN_MAX_BONUS_LEN() instead.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes openzfs#13002 
Closes openzfs#13015

module/zfs/dnode.c

@@ -342,20 +342,11 @@ dnode_byteswap(dnode_phys_t *dnp)
 	 * dnode dnode is smaller than a regular dnode.
 	 */
 	if (dnp->dn_bonuslen != 0) {
-		/*
-		 * Note that the bonus length calculated here may be
-		 * longer than the actual bonus buffer.  This is because
-		 * we always put the bonus buffer after the last block
-		 * pointer (instead of packing it against the end of the
-		 * dnode buffer).
-		 */
-		int off = (dnp->dn_nblkptr-1) * sizeof (blkptr_t);
-		int slots = dnp->dn_extra_slots + 1;
-		size_t len = DN_SLOTS_TO_BONUSLEN(slots) - off;
 		dmu_object_byteswap_t byteswap;
 		ASSERT(DMU_OT_IS_VALID(dnp->dn_bonustype));
 		byteswap = DMU_OT_BYTESWAP(dnp->dn_bonustype);
-		dmu_ot_byteswap[byteswap].ob_func(dnp->dn_bonus + off, len);
+		dmu_ot_byteswap[byteswap].ob_func(DN_BONUS(dnp),
+		    DN_MAX_BONUS_LEN(dnp));
 	}
 
 	/* Swap SPILL block if we have one */