nvmet: make ver stable once connection established

Once some host has connected to the nvmf target, make sure that the
version number is stable and cannot be changed.

Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Noam Gottlieb <ngottlieb@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>