nvme: only set reserved_tags in nvme_alloc_io_tag_set for fabrics controllers

The reserved_tags are only needed for fabrics controllers.  Right now only
fabrics drivers call this helper, so this is harmless, but we'll use it
in the PCIe driver soon.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>