CVE-2026-31503: udp: Fix wildcard bind conflict check when using hash2

In the Linux kernel, the following vulnerability has been resolved:

udp: Fix wildcard bind conflict check when using hash2

When binding a udpsock to a local address and port, UDP uses two hashes (udptable->hash and udptable->hash2) for collision detection. The current code switches to "hash2" when hslot->count > 10.

"hash2" is keyed by local address and local port. "hash" is keyed by local port only.

The issue can be shown in the following bind sequence (pseudo code):

bind(fd1, "[fd00::1]:8888") bind(fd2, "[fd00::2]:8888") bind(fd3, "[fd00::3]:8888") bind(fd4, "[fd00::4]:8888") bind(fd5, "[fd00::5]:8888") bind(fd6, "[fd00::6]:8888") bind(fd7, "[fd00::7]:8888") bind(fd8, "[fd00::8]:8888") bind(fd9, "[fd00::9]:8888") bind(fd10, "[fd00::10]:8888")

/ Correctly return -EADDRINUSE because "hash" is used instead of "hash2". udpliblportinuse() detects the conflict. / bind(failfd, "[::]:8888")

/ After one more socket is bound to "[fd00::11]:8888", hslot->count exceeds 10 and "hash2" is used instead. / bind(fd11, "[fd00::11]:8888") bind(failfd, "[::]:8888") / succeeds unexpectedly /

The same issue applies to the IPv4 wildcard address "0.0.0.0" and the IPv4-mapped wildcard address "::ffff:0.0.0.0". For example, if there are existing sockets bound to "192.168.1.[1-11]:8888", then binding "0.0.0.0:8888" or "[::ffff:0.0.0.0]:8888" can also miss the conflict when hslot->count > 10.

TCP inetcskgetport() already has the correct check in inetusebhash2onbind(). Rename it to inetusehash2onbind() and move it to inethashtables.h so udp.c can reuse it in this fix.