firewire: fix potential uaf in outbound_phy_packet_callback()

&e->event and e point to the same address, and &e->event could
be freed in queue_event. So there is a potential uaf issue if
we dereference e after calling queue_event(). Fix this by adding
a temporary variable to maintain e->client in advance, this can
avoid the potential uaf issue.

Cc: <[e-mail]>
Signed-off-by: Chengfeng Ye <[e-mail]>
Signed-off-by: Takashi Sakamoto <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

drivers/firewire/core-cdev.c

···
 {
 	struct outbound_phy_packet_event *e =
 		container_of(packet, struct outbound_phy_packet_event, p);
+	struct client *e_client;
 
 	switch (status) {
 	/* expected: */
···
 	}
 	e->phy_packet.data[0] = packet->timestamp;
 
+	e_client = e->client;
 	queue_event(e->client, &e->event, &e->phy_packet,
 		    sizeof(e->phy_packet) + e->phy_packet.length, NULL, 0);
-	client_put(e->client);
+	client_put(e_client);
 }
 
 static int ioctl_send_phy_packet(struct client *client, union ioctl_arg *arg)