CVE-2026-43090: xfrm: fix refcount leak in xfrm_migrate_policy_find

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

xfrm: fix refcount leak in xfrmmigratepolicyfind

syzkaller reported a memory leak in xfrmpolicyalloc:

BUG: memory leak unreferenced object 0xffff888114d79000 (size 1024): comm "syz.1.17", pid 931 ... xfrmpolicyalloc+0xb3/0x4b0 net/xfrm/xfrmpolicy.c:432

The root cause is a double call to xfrmpolholdrcu() in xfrmmigratepolicyfind(). The lookup function already returns a policy with held reference, making the second call redundant.

Remove the redundant xfrmpolholdrcu() call to fix the refcount imbalance and prevent the memory leak.

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