nvmecontrol: Use active nslist to enumerate namespaces

Rather than probing all namespace IDs up to cdata.nn for the devlist
command, fetch the active namespace list and iterate over that. This
can be much quicker on Fabrics controllers which often advertise a
large cdata.nn value to support adding additional namespaces at
runtime.

Reviewed by: chuck
Reported by: Neven Z <nevenzfr@gmail.com>
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D47355