mptcp: clean up harmless false expressions

entry->addr.id is u8 with a range from 0 to 255 and MAX_ADDR_ID is 255.
We should drop both false expressions of (entry->addr.id > MAX_ADDR_ID).

We should also remove the obsolete parentheses in the first if branch.

Use U8_MAX for MAX_ADDR_ID and add a comment to show the link to
mptcp_addr_info.id as suggested by Mr. Matthieu Baerts.

Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jean Sacren <sakiwit@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/pm_netlink.c

@@ -38,7 +38,8 @@ struct mptcp_pm_add_entry {
 	u8			retrans_times;
 };
 
-#define MAX_ADDR_ID		255
+/* max value of mptcp_addr_info.id */
+#define MAX_ADDR_ID		U8_MAX
 #define BITMAP_SZ DIV_ROUND_UP(MAX_ADDR_ID + 1, BITS_PER_LONG)
 
 struct pm_nl_pernet {
@@ -825,14 +826,13 @@ static int mptcp_pm_nl_append_new_local_addr(struct pm_nl_pernet *pernet,
 		entry->addr.id = find_next_zero_bit(pernet->id_bitmap,
 						    MAX_ADDR_ID + 1,
 						    pernet->next_id);
-		if ((!entry->addr.id || entry->addr.id > MAX_ADDR_ID) &&
-		    pernet->next_id != 1) {
+		if (!entry->addr.id && pernet->next_id != 1) {
 			pernet->next_id = 1;
 			goto find_next;
 		}
 	}
 
-	if (!entry->addr.id || entry->addr.id > MAX_ADDR_ID)
+	if (!entry->addr.id)
 		goto out;
 
 	__set_bit(entry->addr.id, pernet->id_bitmap);