ANDROID: binder: remove WARN() for redundant txn error

binder_send_failed_reply() is called when a synchronous
transaction fails. It reports an error to the thread that
is waiting for the completion. Given that the transaction
is synchronous, there should never be more than 1 error
response to that thread -- this was being asserted with
a WARN().

However, when exercising the driver with syzbot tests, cases
were observed where multiple "synchronous" requests were
sent without waiting for responses, so it is possible that
multiple errors would be reported to the thread. This testing
was conducted with panic_on_warn set which forced the crash.

This is easily reproduced by sending back-to-back
"synchronous" transactions without checking for any
response (eg, set read_size to 0):

    bwr.write_buffer = (uintptr_t)&bc1;
    bwr.write_size = sizeof(bc1);
    bwr.read_buffer = (uintptr_t)&br;
    bwr.read_size = 0;
    ioctl(fd, BINDER_WRITE_READ, &bwr);
    sleep(1);
    bwr2.write_buffer = (uintptr_t)&bc2;
    bwr2.write_size = sizeof(bc2);
    bwr2.read_buffer = (uintptr_t)&br;
    bwr2.read_size = 0;
    ioctl(fd, BINDER_WRITE_READ, &bwr2);
    sleep(1);

The first transaction is sent to the servicemanager and the reply
fails because no VMA is set up by this client. After
binder_send_failed_reply() is called, the BINDER_WORK_RETURN_ERROR
is sitting on the thread's todo list since the read_size was 0 and
the client is not waiting for a response.

The 2nd transaction is sent and the BINDER_WORK_RETURN_ERROR has not
been consumed, so the thread's reply_error.cmd is still set (normally
cleared when the BINDER_WORK_RETURN_ERROR is handled). Therefore
when the servicemanager attempts to reply to the 2nd failed
transaction, the error is already set and it triggers this warning.

This is a user error since it is not waiting for the synchronous
transaction to complete. If it ever does check, it will see an
error.

Changed the WARN() to a pr_warn().

Signed-off-by: Todd Kjos <tkjos@android.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>