nvmf: Refactor nvmf_add_namespaces to be more generic

Rename to nvmf_scan_active_namespaces and accept an additional
callback function and callback argument. The callback is invoked on
each active namespace enumerated by the active namespace list from the
IDENTIFY command.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D45458