l2tp: Fix memleak in l2tp_udp_encap_recv().

l2tp: Fix memleak in l2tp_udp_encap_recv().

syzbot reported memleak of struct l2tp_session, l2tp_tunnel,
sock, etc. [0]

The cited commit moved down the validation of the protocol
version in l2tp_udp_encap_recv().

The new place requires an extra error handling to avoid the
memleak.

Let's call l2tp_session_put() there.

[0]:
BUG: memory leak
unreferenced object 0xffff88810a290200 (size 512):
  comm "syz.0.17", pid 6086, jiffies 4294944299
  hex dump (first 32 bytes):
    7d eb 04 0c 00 00 00 00 01 00 00 00 00 00 00 00  }...............
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace (crc babb6a4f):
    kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
    slab_post_alloc_hook mm/slub.c:4958 [inline]
    slab_alloc_node mm/slub.c:5263 [inline]
    __do_kmalloc_node mm/slub.c:5656 [inline]
    __kmalloc_noprof+0x3e0/0x660 mm/slub.c:5669
    kmalloc_noprof include/linux/slab.h:961 [inline]
    kzalloc_noprof include/linux/slab.h:1094 [inline]
    l2tp_session_create+0x3a/0x3b0 net/l2tp/l2tp_core.c:1778
    pppol2tp_connect+0x48b/0x920 net/l2tp/l2tp_ppp.c:755
    __sys_connect_file+0x7a/0xb0 net/socket.c:2089
    __sys_connect+0xde/0x110 net/socket.c:2108
    __do_sys_connect net/socket.c:2114 [inline]
    __se_sys_connect net/socket.c:2111 [inline]
    __x64_sys_connect+0x1c/0x30 net/socket.c:2111
    do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
    do_syscall_64+0xa4/0xf80 arch/x86/entry/syscall_64.c:94
    entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: 364798056f518 ("l2tp: Support different protocol versions with same IP/port quadruple")
Reported-by: syzbot+2c42ea4485b29beb0643@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/696693f2.a70a0220.245e30.0001.GAE@google.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/20260113185446.2533333-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>