This fixes an oversight that allowed in-capsule data block SGLs to
potentially refer to more than the received in-capsule data size.
It also makes spdk_nvmf_request_prep_data() less dependent on the
RDMA-specific rx_desc/tx_desc structures.
Change-Id: I34d61aca4cf5ba033849673116d16ec90488dcd4
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>