src: add comment support when adding tables

Adds userdata building logic if a comment is specified when creating a
new table. Adds netlink userdata parsing callback function.

Relies on kernel supporting userdata for nft_table.

Example:

> nft add table ip x { comment "test"\; }
> nft list ruleset

table ip x {
	comment "test"
}

Signed-off-by: Jose M. Guisado Gomez <guigom@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

2 files changed, 8 insertions, 0 deletions

diff --git a/tests/shell/testcases/optionals/comments_table_0 b/tests/shell/testcases/optionals/comments_table_0
new file mode 100755
index 00000000..a0dfd749
--- /dev/null
+++ b/tests/shell/testcases/optionals/comments_table_0
@@ -0,0 +1,5 @@
+#!/bin/bash
+
+# comments are shown
+
+$NFT add table test { comment \"test_comment\"\; }
diff --git a/tests/shell/testcases/optionals/dumps/comments_table_0.nft b/tests/shell/testcases/optionals/dumps/comments_table_0.nft
new file mode 100644
index 00000000..32ae3c2d
--- /dev/null
+++ b/tests/shell/testcases/optionals/dumps/comments_table_0.nft
@@ -0,0 +1,3 @@
+table ip test {
+	comment "test_comment"
+}