nvmf: Add infrastructure kernel module for NVMe over Fabrics

nvmf_transport.ko provides routines for managing NVMeoF queue pairs
and capsules. It provides a glue layer between transports (such as
TCP or RDMA) and an NVMeoF host (initiator) and controller (target).

Unlike the synchronous API exposed to the host and controller by
libnvmf, the kernel's transport layer uses an asynchronous API built
on callbacks. Upper layers provide callbacks on queue pairs that are
invoked for transport errors (error_cb) or anytime a capsule is
received (receive_cb).

Data transfers for a command are usually associated with a callback
that is invoked once a transfer has finished either due to an error
or successful completion.

For an upper layer that is a host, command capsules are allocated and
populated with an NVMe SQE by calling nvmf_allocate_command. A data
buffer (described by a struct memdesc) can be associated with a
command capsule before it is transmitted via nvmf_capsule_append_data.
This function accepts a direction (send vs receive) as well as the
data transfer callback. The host then transmits the command via
nvmf_transmit_capsule. The host must ensure that the data buffer
described by the 'struct memdesc' remains valid until the data
transfer callback is called. The queue pair's receive_cb callback
should match received response capsules up with previously transmitted
commands.

For the controller, incoming commands are received via the queue
pair's receive_cb callback. nvmf_receive_controller_data is used to
retrieve any data from a command (e.g. the data for a WRITE command).
It can be called multiple times to split the data transfer into
smaller sizes. This function accepts an I/O completion callback that
is invoked once the data transfer has completed.
nvmf_send_controller_data is used to send data to a remote host in
response to a command. In this case a callback function is not used
but the status is returned synchronously. Finally, the controller can
allocate a response capsule via nvmf_allocate_response populated with
a supplied CQE and send the response via nvmf_transmit_capsule.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44711