nvme: Have NVME_FABRICS select NVME_CORE instead of transport drivers

Transport drivers need both core and fabrics modules, instead of
selecting both, have the selection transitive such that NVME_FABRICS
selects NVME_CORE and transport drivers select NVME_FABRICS.

Suggested-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>