binder: add missing binder_unlock()

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
a60b890f607d ("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>