net: qrtr: Fix a refcount bug in qrtr_recvmsg()

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

net: qrtr: Fix a refcount bug in qrtr_recvmsg()

Syzbot reported a bug as following:

refcount_t: addition on 0; use-after-free.
...
RIP: 0010:refcount_warn_saturate+0x17c/0x1f0 lib/refcount.c:25
...
Call Trace:
<TASK>
__refcount_add include/linux/refcount.h:199 [inline]
__refcount_inc include/linux/refcount.h:250 [inline]
refcount_inc include/linux/refcount.h:267 [inline]
kref_get include/linux/kref.h:45 [inline]
qrtr_node_acquire net/qrtr/af_qrtr.c:202 [inline]
qrtr_node_lookup net/qrtr/af_qrtr.c:398 [inline]
qrtr_send_resume_tx net/qrtr/af_qrtr.c:1003 [inline]
qrtr_recvmsg+0x85f/0x990 net/qrtr/af_qrtr.c:1070
sock_recvmsg_nosec net/socket.c:1017 [inline]
sock_recvmsg+0xe2/0x160 net/socket.c:1038
qrtr_ns_worker+0x170/0x1700 net/qrtr/ns.c:688
process_one_work+0x991/0x15c0 kernel/workqueue.c:2390
worker_thread+0x669/0x1090 kernel/workqueue.c:2537

It occurs in the concurrent scenario of qrtr_recvmsg() and
qrtr_endpoint_unregister() as following:

cpu0 cpu1
qrtr_recvmsg qrtr_endpoint_unregister
qrtr_send_resume_tx qrtr_node_release
qrtr_node_lookup mutex_lock(&qrtr_node_lock)
spin_lock_irqsave(&qrtr_nodes_lock, ) refcount_dec_and_test(&node->ref) [node->ref == 0]
radix_tree_lookup [node != NULL] __qrtr_node_release
qrtr_node_acquire spin_lock_irqsave(&qrtr_nodes_lock, )
kref_get(&node->ref) [WARNING] ...
mutex_unlock(&qrtr_node_lock)

Use qrtr_node_lock to protect qrtr_node_lookup() implementation, this
is actually improving the protection of node reference.