socket-util: introduce tos_to_priority()

This maps from TOS, which can be used for setsockopt(IPPROTO_IP, IP_TOS),
to socket priority, which can be used for setsockopt(SOL_SOCKET, SO_PRIORITY).

With this, we can set priority like the following:
```
uint8_t tos = IPTOS_CLASS_CS6;
setsockopt_int(fd, IPPROTO_IP, IP_TOS, tos);
setsockopt_int(fd, SOL_SOCKET, SO_PRIORITY, tos_to_priority(tos));
```

Co-authored with Google Gemini.

src/basic/socket-util.c

@@ -3,6 +3,7 @@
 #include <fcntl.h>
 #include <linux/if.h>
 #include <linux/if_arp.h>
+#include <linux/pkt_sched.h>
 #include <mqueue.h>
 #include <net/if.h>
 #include <netdb.h>
@@ -1873,3 +1874,29 @@ void cmsg_close_all(struct msghdr *mh) {
                 }
         }
 }
+
+int tos_to_priority(uint8_t tos) {
+        /* Map the IP Precedence (top 3 bits of the TOS field) to Linux internal packet priorities
+         * (TC_PRIO_*). This exactly mirrors the standard Linux kernel IP precedence-to-priority mapping
+         * (rt_tos2priority) to ensure consistent behavior when explicitly setting SO_PRIORITY. */
+        switch (IPTOS_PREC(tos)) {
+        case IPTOS_PREC_NETCONTROL:      /* 0xc0 (CS7) - Network Control. Used for infrastructure control (e.g., STP, keepalives). */
+        case IPTOS_PREC_INTERNETCONTROL: /* 0xe0 (CS6) - Internetwork Control. Used for routing protocols (e.g., OSPF, BGP) and DHCP. */
+                return TC_PRIO_CONTROL;
+
+        case IPTOS_PREC_CRITIC_ECP:      /* 0xa0 (CS5) - Critical. Used for delay-sensitive traffic like Voice over IP (VoIP). */
+        case IPTOS_PREC_FLASHOVERRIDE:   /* 0x80 (CS4) - Flash Override. Used for interactive video and multimedia. */
+                return TC_PRIO_INTERACTIVE;
+
+        case IPTOS_PREC_FLASH:           /* 0x60 (CS3) - Flash. Used for broadcast video and call signaling (e.g., SIP). */
+        case IPTOS_PREC_IMMEDIATE:       /* 0x40 (CS2) - Immediate. Used for OAM (Operations, Administration, and Management) and transactional data. */
+                return TC_PRIO_INTERACTIVE_BULK;
+
+        case IPTOS_PREC_PRIORITY:        /* 0x20 (CS1) - Priority. Used for background traffic and bulk data transfers. */
+                return TC_PRIO_BULK;
+
+        case IPTOS_PREC_ROUTINE:         /* 0x00 (CS0) - Routine. Best effort traffic. */
+        default:
+                return TC_PRIO_BESTEFFORT;
+        }
+}

src/basic/socket-util.h

@@ -270,3 +270,5 @@ int netlink_socket_get_multicast_groups(int fd, size_t *ret_len, uint32_t **ret_
 int socket_get_cookie(int fd, uint64_t *ret);
 
 void cmsg_close_all(struct msghdr *mh);
+
+int tos_to_priority(uint8_t tos);