usbip: usbip_host: fix bad unlock balance during stub_probe()

[Shuah Khan <shuah@kernel.org>]

stub_probe() calls put_busid_priv() in an error path when device isn't
found in the busid_table. Fix it by making put_busid_priv() safe to be
called with null struct bus_id_priv pointer.

This problem happens when "usbip bind" is run without loading usbip_host
driver and then running modprobe. The first failed bind attempt unbinds
the device from the original driver and when usbip_host is modprobed,
stub_probe() runs and doesn't find the device in its busid table and calls
put_busid_priv(0 with null bus_id_priv pointer.

usbip-host 3-10.2: 3-10.2 is not in match_busid table...  skip!

[  367.359679] =====================================
[  367.359681] WARNING: bad unlock balance detected!
[  367.359683] 4.17.0-rc4+ #5 Not tainted
[  367.359685] -------------------------------------
[  367.359688] modprobe/2768 is trying to release lock (
[  367.359689]

==================================================================
[  367.359696] BUG: KASAN: null-ptr-deref in
print_unlock_imbalance_bug+0x99/0x110
[  367.359699] Read of size 8 at addr 0000000000000058 by task
modprobe/2768

[  367.359705] CPU: 4 PID: 2768 Comm: modprobe Not tainted 4.17.0-rc4+ #5

Fixes: 22076557b07c ("usbip: usbip_host: fix NULL-ptr deref and
use-after-free errors") in usb-linus

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>
---

 drivers/usb/usbip/stub_main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/usbip/stub_main.c b/drivers/usb/usbip/stub_main.c
index 41c7b9de2a92..bf8a5feb0ee9 100644
--- a/drivers/usb/usbip/stub_main.c
+++ b/drivers/usb/usbip/stub_main.c
@@ -82,7 +82,8 @@ struct bus_id_priv *get_busid_priv(const char *busid)
 
 void put_busid_priv(struct bus_id_priv *bid)
 {
-	spin_unlock(&bid->busid_lock);
+	if (bid)
+		spin_unlock(&bid->busid_lock);
 }
 
 static int add_match_busid(char *busid)