tcp: unloading a module that is set to default should error.
I just discovered that the return of the EBUSY error was incorrectly rigged so that you could unload a CC module that was set to default. Its supposed to be an EBUSY error. Make it so. Reviewed by: Michael Tuexen Sponsored by: Netflix Inc. Differential Revision: https://reviews.freebsd.org/D33229