change delete operation to set for restoring direntry in nfs_kv_remove

https://github.com/vitalif/vitastor/pull/95

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src/nfs/nfs_kv_remove.cpp

@@ -199,7 +199,7 @@ resume_4:
         else
         {
             // Not OK, restore direntry
-            st->self->parent->db->del(kv_direntry_key(st->dir_ino, st->filename), [st](int res)
+            st->self->parent->db->set(kv_direntry_key(st->dir_ino, st->filename), st->direntry_text, [st](int res)
             {
                 st->res2 = res;
                 nfs_kv_continue_delete(st, 5);