rust: dma_fence: gate abstractions on CONFIG_DMA_SHARED_BUFFER

The dma_fence abstractions added by commit 3282d59160 ("rust: Add
dma_fence abstractions") are compiled unconditionally, but the C
`dma_fence_*` symbols they reference live in
drivers/dma-buf/dma-fence.c, which is only built when
CONFIG_DMA_SHARED_BUFFER=y. Kernels with CONFIG_RUST=y and
CONFIG_RUST_KERNEL_DOCTESTS=y but CONFIG_DMA_SHARED_BUFFER=n fail
to link:

  ld.lld: error: undefined symbol: dma_fence_release
  >>> referenced by usercopy_64.c
  >>>               vmlinux.o:(rust_helper_dma_fence_put)
  ... dma_fence_signal{,_locked}, dma_fence_context_alloc,
      dma_fence_init, dma_fence_{add,remove}_callback ...

Wrap the dma_fence.c helper include in the same
`pub mod dma_buf;` in rust/kernel/lib.rs with a matching #[cfg].

Fixes: 3282d59160 ("rust: Add dma_fence abstractions")
Signed-off-by: Edwin Peer <epeer@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Philipp Stanner <phasta@kernel.org>
Link: https://patch.msgid.link/20260909202327.33285-1-epeer@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/helpers/helpers.c

@@ -57,8 +57,8 @@
 #include "cred.c"
 #include "device.c"
 #include "dma.c"
-#include "dma_fence.c"
 #ifdef CONFIG_DMA_SHARED_BUFFER
+#include "dma_fence.c"
 #include "dma-resv.c"
 #endif
 #include "drm.c"

rust/kernel/lib.rs

@@ -67,6 +67,7 @@
 pub mod device_id;
 pub mod devres;
 pub mod dma;
+#[cfg(CONFIG_DMA_SHARED_BUFFER)]
 pub mod dma_buf;
 pub mod driver;
 #[cfg(CONFIG_DRM = "y")]