zvol: remove unused variable

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes openzfs#12917

module/zfs/zvol.c

@@ -1311,13 +1311,12 @@ static void
 zvol_rename_minors_impl(const char *oldname, const char *newname)
 {
 	zvol_state_t *zv, *zv_next;
-	int oldnamelen, newnamelen;
+	int oldnamelen;
 
 	if (zvol_inhibit_dev)
 		return;
 
 	oldnamelen = strlen(oldname);
-	newnamelen = strlen(newname);
 
 	rw_enter(&zvol_state_lock, RW_READER);