Remove hard coded number of lun definition

The number of lun exposed is now exposed via sysctl by the kernel.
Use that number in ctlstat instead of the hardcoded version
Add a backward compatibility in case the sysctl(2) request fails.

This also allows ctlstat -l 1118 to actually work when having more than
1024 luns.

Reviewed by: avg, manu (both before the backward compatibility addition)
Approved by: avg, manu (both before the backward compatibility addition)
MFC after: 2 weeks
Sponsored by: Gandi.net
Differential Revision: https://reviews.freebsd.org/D13446