Fix decoding of BPF_MAP_UPDATE_ELEM flags

* bpf.c (bpf_map_update_elem): Print attr.flags using printxval64.
diff --git a/bpf.c b/bpf.c
index f7156f0..20070b3 100644
--- a/bpf.c
+++ b/bpf.c
@@ -83,7 +83,7 @@
 	printfd(tcp, attr.map_fd);
 	tprintf(", key=%#" PRIx64 ", value=%#" PRIx64 ", flags=",
 		attr.key, attr.value);
-	printxval(bpf_map_update_elem_flags, attr.flags, "BPF_???");
+	printxval64(bpf_map_update_elem_flags, attr.flags, "BPF_???");
 	tprints("}");
 }