binder: add missing binder_unlock()

BugLink: http://bugs.launchpad.net/bugs/1762453

When commit 4be5a281 ("binder: check for binder_thread allocation
failure in binder_poll()") was applied to 4.4-stable and 4.9-stable it
was forgotten to release the global binder lock in the new error path.
The global binder lock wasn't removed until v4.14, by commit
a60b890f ("binder: remove global binder lock").

Fix the new error path to release the lock.
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

drivers/android/binder.c

@@ -2622,8 +2622,10 @@ static unsigned int binder_poll(struct file *filp,
 	binder_lock(__func__);
 
 	thread = binder_get_thread(proc);
-	if (!thread)
+	if (!thread) {
+		binder_unlock(__func__);
 		return POLLERR;
+	}
 
 	wait_for_proc_work = thread->transaction_stack == NULL &&
 		list_empty(&thread->todo) && thread->return_error == BR_OK;