If it cannot be allocated, we should return error. Change-Id: I48aa50a8842c35ee112fe7185128b1bc1930176e Signed-off-by: Ziye Yang <optimistyzy@gmail.com> Reviewed-on: https://review.gerrithub.io/386369 Tested-by: SPDK Automated Test System <sys_sgsw@intel.com> Reviewed-by: Jim Harris <james.r.harris@intel.com> Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com> Reviewed-by: Ben Walker <benjamin.walker@intel.com>