Without CONFIG_DAX_DRIVER, drivers/md/dm-stripe.c fails to compile:

drivers/md/dm-stripe.c:515:27: error: 'stripe_dax_memcpy_toiovecend' undeclared here (not in a function)
.dax_memcpy_toiovecend = stripe_dax_memcpy_toiovecend,
^

CONFIG_DAX_DRIVER will be automaticallly selected by NVDIMM support,
which needs PHYS_ADDR_T_64BIT setup.

diff -up ./drivers/md/dm-stripe.c.dax ./drivers/md/dm-stripe.c
--- ./drivers/md/dm-stripe.c.dax	2018-10-05 05:18:19.000000000 +0900
+++ ./drivers/md/dm-stripe.c	2018-11-01 12:53:35.000000000 +0900
@@ -370,6 +370,7 @@ static int stripe_dax_memcpy_toiovecend(
 #define stripe_dax_direct_access NULL
 #define stripe_dax_memcpy_fromiovecend NULL
 #define stripe_dax_memcpy_fromiovecend NULL
+#define stripe_dax_memcpy_toiovecend NULL
 #endif
 
 /*