nvmf: Fix an off by one error when scanning active namespace IDs

The active namespace list query fetches namespaces greater than the
passed in namespace ID, not greater than or equal to the passed in
namespace ID. Thus, a multi-page request should start with the last
namespace ID from the previous page, not that ID plus 1.

While here, make use of NVME_GLOBAL_NAMESPACE_TAG instead of a magic
number to handle the edge case that the last namespace ID in a page is
the largest valid namespace ID.

Reviewed by: chuck
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D47393