CVE-2025-21680: pktgen: Avoid out-of-bounds access in get_imix_entries

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

pktgen: Avoid out-of-bounds access in getimixentries

Passing a sufficient amount of imix entries leads to invalid access to the pktdev->imixentries array because of the incorrect boundary check.

UBSAN: array-index-out-of-bounds in net/core/pktgen.c:874:24 index 20 is out of range for type 'imixpkt [20]' CPU: 2 PID: 1210 Comm: bash Not tainted 6.10.0-rc1 #121 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996) Call Trace: <TASK> dumpstacklvl lib/dumpstack.c:117 ubsanhandleoutofbounds lib/ubsan.c:429 getimixentries net/core/pktgen.c:874 pktgenifwrite net/core/pktgen.c:1063 pdewrite fs/proc/inode.c:334 procregwrite fs/proc/inode.c:346 vfswrite fs/readwrite.c:593 ksyswrite fs/readwrite.c:644 dosyscall64 arch/x86/entry/common.c:83 entrySYSCALL64afterhwframe arch/x86/entry/entry64.S:130

Found by Linux Verification Center (linuxtesting.org) with SVACE.

[ fp: allow to fill the array completely; minor changelog cleanup ]