net/mctp: publish the mctp_dev only after it is initialised

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

net/mctp: publish the mctp_dev only after it is initialised

mctp_add_dev() links the new mctp_dev into dev->mctp_ptr before mdev->dev
is assigned, so a reader that picks the device up in that window
dereferences a net_device pointer that is still NULL:

KASAN: null-ptr-deref in range [0x00000000000000b0-0x00000000000000b7]
Call Trace:
<TASK>
? __pfx_mctp_sendmsg (./include/linux/sockptr.h:49)
? __pfx_mctp_dst_output (net/mctp/route.c:41)
? selinux_socket_sendmsg (security/selinux/hooks.c:5278)
____sys_sendmsg (net/socket.c:775 (discriminator 1) net/socket.c:790
(discriminator 1) net/socket.c:2684 (discriminator 1))
? __pfx_____sys_sendmsg (net/socket.c:1131)
? __pfx_copy_msghdr_from_user (net/socket.c:2590)
? update_cfs_rq_load_avg (kernel/sched/fair.c:5478)
___sys_sendmsg (net/socket.c:2738)
? __pfx____sys_sendmsg (net/socket.c:2625)
? perf_event_task_tick (./include/linux/rcupdate.h:838)
? sched_tick (kernel/sched/core.c:5818)
? clockevents_program_event (kernel/time/clockevents.c:372)
? fdget (./include/linux/rcupdate.h:873 fs/file.c:1100)
__sys_sendmsg (net/socket.c:2770)
? __pfx___sys_sendmsg (net/socket.c:2751)
do_syscall_64 (arch/x86/entry/syscall_64.c:63)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
</TASK>

Fix by publishing only once the object is fully initialised:
__mctp_dev_get() runs under rcu_read_lock() only and hands
this object to readers as soon as mctp_ptr is assigned, so
they must never observe mdev->dev == NULL.