iscsi: fix iSER compilation

This fails in Fedora 28.
Reported-by: NAndreas Schwab <schwab@suse.de>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/iscsi.c

@@ -2244,7 +2244,7 @@ static BlockDriver bdrv_iser = {
     .create_opts            = &iscsi_create_opts,
     .bdrv_reopen_prepare    = iscsi_reopen_prepare,
     .bdrv_reopen_commit     = iscsi_reopen_commit,
-    .bdrv_invalidate_cache  = iscsi_invalidate_cache,
+    .bdrv_co_invalidate_cache  = iscsi_co_invalidate_cache,
 
     .bdrv_getlength  = iscsi_getlength,
     .bdrv_get_info   = iscsi_get_info,