nvme-fabrics: get a reference when reusing a nvme_host structure

Without this we'll get a use after free after connecting two controller
using the same hostnqn and then disconnecting one of them.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jay Freyensee <james_p_freyensee@linux.intel.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>