The parent field is in the cache line of nvme_request that is only
supposed to be accessed for split (child) I/Os. All accesses to parent
are done from child-specific calls now, so it does not need to be
initialized in the common case of a non-split I/O.
nvme_request_add_child() will set parent when splitting occurs.
Change-Id: Ib86c16ba1ea2ce32f62079831101da2a099047af
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>