kernel-netlink: Allow Netlink send buffer size to be configured via compile option

The receive buffer size can already be changed via strongswan.conf if
necessary.

src/libhydra/plugins/kernel_netlink/kernel_netlink_shared.h

@@ -20,15 +20,23 @@
 
 #include <linux/rtnetlink.h>
 
+/**
+ * Default buffer size.
+ *
+ * 1024 byte is currently sufficient for all operations.
+ */
+#ifndef KERNEL_NETLINK_BUFSIZE
+#define KERNEL_NETLINK_BUFSIZE 1024
+#endif
+
 /**
  * General purpose netlink buffer.
  *
- * 1024 byte is currently sufficient for all operations. Some platform
- * require an enforced aligment to four bytes (e.g. ARM).
+ * Some platforms require an enforced aligment to four bytes (e.g. ARM).
  */
 typedef union {
 	struct nlmsghdr hdr;
-	u_char bytes[1024];
+	u_char bytes[KERNEL_NETLINK_BUFSIZE];
 } netlink_buf_t __attribute__((aligned(RTA_ALIGNTO)));
 
 typedef struct netlink_socket_t netlink_socket_t;