nvme_rdma_req_get() is an internal function, and its only caller already
checks for a valid rqpair, so the NULL check is unnecessary.
Also clean up the redundant STAILQ_EMPTY/STAILQ_FIRST logic and use
STAILQ_REMOVE_HEAD.
Change-Id: Ic3828e8b5e881879173cb59350e39c5fac90e6ef
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>