xen: change xen/[gntdev/gntalloc] to default m

When CONFIG_XEN is enabled the gntdev and gntalloc driver will be
compiled as a module by default.

[v2: Added the fix for the gntalloc driver as well]
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/Kconfig

@@ -76,6 +76,7 @@ config XEN_XENBUS_FRONTEND
 config XEN_GNTDEV
 	tristate "userspace grant access device driver"
 	depends on XEN
+	default m
 	select MMU_NOTIFIER
 	help
 	  Allows userspace processes to use grants.
@@ -83,6 +84,7 @@ config XEN_GNTDEV
 config XEN_GRANT_DEV_ALLOC
 	tristate "User-space grant reference allocator driver"
 	depends on XEN
+	default m
 	help
 	  Allows userspace processes to create pages with access granted
 	  to other domains. This can be used to implement frontend drivers