dma-mapping: add missing inline for dma_free_attrs

Under an UML build for an upcoming series [1], I got `-Wstatic-in-inline`
for `dma_free_attrs`:

      BINDGEN rust/bindings/bindings_generated.rs - due to target missing
    In file included from rust/helpers/helpers.c:59:
    rust/helpers/dma.c:17:2: warning: static function 'dma_free_attrs' is used in an inline function with external linkage [-Wstatic-in-inline]
       17 |         dma_free_attrs(dev, size, cpu_addr, dma_handle, attrs);
          |         ^
    rust/helpers/dma.c:12:1: note: use 'static' to give inline function 'rust_helper_dma_free_attrs' internal linkage
       12 | __rust_helper void rust_helper_dma_free_attrs(struct device *dev, size_t size,
          | ^
          | static

The issue is that `dma_free_attrs` was not marked `inline` when it was
introduced alongside the rest of the stubs.

Thus mark it.

Fixes: ed6ccf1 ("dma-mapping: properly stub out the DMA API for !CONFIG_HAS_DMA")
Closes: https://lore.kernel.org/rust-for-linux/[email protected]/ [1]
Signed-off-by: Miguel Ojeda <[email protected]>
Signed-off-by: Marek Szyprowski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ojeda

include/linux/dma-mapping.h

@@ -255,8 +255,8 @@ static inline void *dma_alloc_attrs(struct device *dev, size_t size,
 {
 	return NULL;
 }
-static void dma_free_attrs(struct device *dev, size_t size, void *cpu_addr,
-		dma_addr_t dma_handle, unsigned long attrs)
+static inline void dma_free_attrs(struct device *dev, size_t size,
+		void *cpu_addr, dma_addr_t dma_handle, unsigned long attrs)
 {
 }
 static inline void *dmam_alloc_attrs(struct device *dev, size_t size,