nvmf: Add a transport hook to limit the size of host data transfers

Smart NIC offload transports may have a cap on the size of the largest
data PDU that can be received. Allow these transports to enforce a
cap on the size of an I/O request submitted by the nvmf(4) host.

NB: The controller is able to advertise a maximum-supported PDU size
during TCP negotiation, but there is no way in the protocol to
advertise a maximum size that the host can receive.

Sponsored by: Chelsio Communications