netlink: Add flag to suppress print of nlmsg error

Allow callers of the dump API to handle nlmsg errors (e.g., an
unsupported feature). Setting RTNL_HANDLE_F_SUPPRESS_NLERR in the
rtnl_handle avoids unnecessary messages to the users in some case.
For example,

  RTNETLINK answers: Operation not supported

when probing for support of a new feature.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>

include/libnetlink.h

@@ -21,6 +21,7 @@ struct rtnl_handle {
 	int			proto;
 	FILE		       *dump_fp;
 #define RTNL_HANDLE_F_LISTEN_ALL_NSID		0x01
+#define RTNL_HANDLE_F_SUPPRESS_NLERR		0x02
 	int			flags;
 };
 

lib/libnetlink.c

@@ -299,7 +299,8 @@ static void rtnl_dump_error(const struct rtnl_handle *rth,
 		     errno == EOPNOTSUPP))
 			return;
 
-		perror("RTNETLINK answers");
+		if (!(rth->flags & RTNL_HANDLE_F_SUPPRESS_NLERR))
+			perror("RTNETLINK answers");
 	}
 }