How about another way of controlling this via sysctl.

First, add code to for read only:
	/proc/sys/net/ipv4/tcp_available_congestion_control  (or shorter name)
this will show all things compiled in (even if not loaded yet). Similar
to /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies

Second, add flag (allowed) to the tcp_congestion structure [inverse of
earlier restricted]

Third, add read-write
	/proc/sys/net/ipv4/tcp_allowed_congestion_control
to show and set/clear the allowed flag. Default value would be
"reno xxx" where xxx is what ever the default value from the kernel
config is (currently cubic).

I would use sysfs for this, but it make sense not to spread TCP stuff into
both sysctl and sysfs.