Fix NULL deref when zvol_alloc() fails

If zvol_alloc() fails zv will be set to NULL and dereferenced
in out_dmu_objset_disown.  To avoid this entirely the zv->objset
line is moved up in to the success block.

Original-patch-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1109

module/zfs/zvol.c

@@ -1306,9 +1306,9 @@ __zvol_create_minor(const char *name)
 	else
 		zil_replay(os, zv, zvol_replay_vector);
 
+	zv->zv_objset = NULL;
 out_dmu_objset_disown:
 	dmu_objset_disown(os, zvol_tag);
-	zv->zv_objset = NULL;
 out_doi:
 	kmem_free(doi, sizeof(dmu_object_info_t));
 out: