nvmf/tcp: execute buffer allocation only if request is the first of pendings

RDMA transport executes spdk_nvmf_rdma_request_parse_sgl() only if
the request is the first of the pending requests in the case
RDMA_REQUEST_STATE_NEED_BUFFER in the state machine
spdk_nvmf_rdma_requests_process().

This made RDMA transport possible to use STAILQ for pending requests
because STAILQ_REMOVE parses from head and is slow when the target is in
the middle of STAILQ.

On the other hand, TCP transport executes spdk_nvmf_tcp_req_parse_sgl()
even if the request is in the middle of the pending request in the case
TCP_REQUEST_STATE_NEED_BUFFER in the state machine
spdk_nvmf_tcp_req_process() if the request has in-capsule data.

Hence TCP transport have used TAILQ for pending requests.

This patch removes the condition if the request has in-capsule data
from the case TCP_REQUEST_STATE_NEED_BUFFER.

The purpose of this patch is to unify I/O buffer management further.

Performance degradation was not observed even after this patch.

Signed-off-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Change-Id: Idc97fe20f7013ca66fd58587773edb81ef7cbbfc
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/466636


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Broadcom SPDK FC-NVMe CI <spdk-ci.pdl@broadcom.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>