nvmf: Permit failing I/O requests while disconnected

Add a kern.nvmf.fail_on_disconnection sysctl similar to the
kern.iscsi.fail_on_disconnection sysctl. This causes pending I/O
requests to fail wi

nvmf: Permit failing I/O requests while disconnected

Add a kern.nvmf.fail_on_disconnection sysctl similar to the
kern.iscsi.fail_on_disconnection sysctl. This causes pending I/O
requests to fail with an error if an association is disconnected
instead of requeueing to be retried once the association is
reconnected. As with iSCSI, the default is to queue and retry
operations.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D45308

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nvmf: The in-kernel NVMe over Fabrics host

This is the client (initiator in SCSI terms) for NVMe over Fabrics.
Userland is responsible for creating a set of queue pairs and then
handing them off via

nvmf: The in-kernel NVMe over Fabrics host

This is the client (initiator in SCSI terms) for NVMe over Fabrics.
Userland is responsible for creating a set of queue pairs and then
handing them off via an ioctl to this driver, e.g. via the 'connect'
command from nvmecontrol(8). An nvmeX new-bus device is created
at the top-level to represent the remote controller similar to PCI
nvmeX devices for PCI-express controllers.

As with nvme(4), namespace devices named /dev/nvmeXnsY are created and
pass through commands can be submitted to either the namespace devices
or the controller device. For example, 'nvmecontrol identify nvmeX'
works for a remote Fabrics controller the same as for a PCI-express
controller.

nvmf exports remote namespaces via nda(4) devices using the new NVMF
CAM transport. nvmf does not support nvd(4), only nda(4).

Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44714

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