virtiofs: add a mount option to enable dax

Add a mount option to allow using dax with virtio_fs.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
diff --git a/fs/fuse/Kconfig b/fs/fuse/Kconfig
index 0156dc8..fddd406 100644
--- a/fs/fuse/Kconfig
+++ b/fs/fuse/Kconfig
@@ -38,3 +38,16 @@
 
 	  If you want to share files between guests or with the host, answer Y
 	  or M.
+
+config FUSE_DAX
+	bool "Virtio Filesystem Direct Host Memory Access support"
+	default y
+	depends on VIRTIO_FS
+	depends on FS_DAX
+	depends on DAX_DRIVER
+	help
+	  This allows bypassing guest page cache and allows mapping host page
+	  cache directly in guest address space.
+
+	  If you want to allow mounting a Virtio Filesystem with the "dax"
+	  option, answer Y.