xen/gntdev: Use select for DMA_SHARED_BUFFER

DMA_SHARED_BUFFER can not be enabled by the user (it represents a library
set in the kernel). The kconfig convention is to use select for such
symbols so they are turned on implicitly when the user enables a kconfig
that needs them.

Otherwise the XEN_GNTDEV_DMABUF kconfig is overly difficult to enable.

Fixes: 932d6562179e ("xen/gntdev: Add initial support for dma-buf UAPI")
Cc: Oleksandr Andrushchenko <[e-mail]>
Cc: Boris Ostrovsky <[e-mail]>
Cc: [e-mail]
Cc: Juergen Gross <[e-mail]>
Cc: Stefano Stabellini <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Reviewed-by: Oleksandr Andrushchenko <[e-mail]>
Signed-off-by: Jason Gunthorpe <[e-mail]>
Signed-off-by: Juergen Gross <[e-mail]>

drivers/xen/Kconfig

···
 
 config XEN_GNTDEV_DMABUF
 	bool "Add support for dma-buf grant access device driver extension"
-	depends on XEN_GNTDEV && XEN_GRANT_DMA_ALLOC && DMA_SHARED_BUFFER
+	depends on XEN_GNTDEV && XEN_GRANT_DMA_ALLOC
+	select DMA_SHARED_BUFFER
 	help
 	  Allows userspace processes and kernel modules to use Xen backed
 	  dma-buf implementation. With this extension grant references to