ax25: Fix refcount leak caused by setting SO_BINDTODEVICE sockopt

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

ax25: Fix refcount leak caused by setting SO_BINDTODEVICE sockopt

If an AX25 device is bound to a socket by setting the SO_BINDTODEVICE

socket option, a refcount leak will occur in ax25_release().

Commit 9fd75b66b8f6 ("ax25: Fix refcount leaks caused by ax25_cb_del()")

added decrement of device refcounts in ax25_release(). In order for that

to work correctly the refcounts must already be incremented when the

device is bound to the socket. An AX25 device can be bound to a socket

by either calling ax25_bind() or setting SO_BINDTODEVICE socket option.

In both cases the refcounts should be incremented, but in fact it is done

only in ax25_bind().

This bug leads to the following issue reported by Syzkaller:

================================================================

refcount_t: decrement hit 0; leaking memory.

WARNING: CPU: 1 PID: 5932 at lib/refcount.c:31 refcount_warn_saturate+0x1ed/0x210 lib/refcount.c:31

Modules linked in:

CPU: 1 UID: 0 PID: 5932 Comm: syz-executor424 Not tainted 6.13.0-rc4-syzkaller-00110-g4099a71718b0 #0

Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014

RIP: 0010:refcount_warn_saturate+0x1ed/0x210 lib/refcount.c:31

Call Trace:

<TASK>

__refcount_dec include/linux/refcount.h:336 [inline]

refcount_dec include/linux/refcount.h:351 [inline]

ref_tracker_free+0x710/0x820 lib/ref_tracker.c:236

netdev_tracker_free include/linux/netdevice.h:4156 [inline]

netdev_put include/linux/netdevice.h:4173 [inline]

netdev_put include/linux/netdevice.h:4169 [inline]

ax25_release+0x33f/0xa10 net/ax25/af_ax25.c:1069

__sock_release+0xb0/0x270 net/socket.c:640

sock_close+0x1c/0x30 net/socket.c:1408

...

do_syscall_x64 arch/x86/entry/common.c:52 [inline]

do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83

entry_SYSCALL_64_after_hwframe+0x77/0x7f

...

</TASK>

================================================================

Fix the implementation of ax25_setsockopt() by adding increment of

refcounts for the new device bound, and decrement of refcounts for

the old unbound device.