kernel: add kmod-dax

In kernel 4.14 kmod-dm depends on kmod-dax.
Add DAX: "Direct access to differentiated memory" to LEDE.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/kernel/linux/modules/block.mk

@@ -206,10 +206,21 @@ endef
 $(eval $(call KernelPackage,block2mtd))
 
 
+define KernelPackage/dax
+  SUBMENU:=$(BLOCK_MENU)
+  TITLE:=DAX: direct access to differentiated memory
+  DEPENDS:=@LINUX_4_14
+  KCONFIG:=CONFIG_DAX
+  FILES:=$(LINUX_DIR)/drivers/dax/dax.ko
+endef
+
+$(eval $(call KernelPackage,dax))
+
+
 define KernelPackage/dm
   SUBMENU:=$(BLOCK_MENU)
   TITLE:=Device Mapper
-  DEPENDS:=+kmod-crypto-manager
+  DEPENDS:=+kmod-crypto-manager +LINUX_4_14:kmod-dax
   # All the "=n" are unnecessary, they're only there
   # to stop the config from asking the question.
   # MIRROR is M because I've needed it for pvmove.