nvmet: introduce flags member in nvmet_fabrics_ops

Replace has_keyed_sgls and metadata_support booleans with a flags member
that will be used for adding more features in the future.

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Reviewed-by: Israel Rukshin <israelr@mellanox.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>