Add the missing unlock before return from function usbip_sockfd_store()
in the error handling case.

Fixes: bd8b8204 ("usbip: vudc synchronize sysfs code paths")
Reported-by: Hulk Robot <hulkci@huawei.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Link: https://lore.kernel.org/r/20210408112305.1022247-1-yebin10@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>