dma-buf: remove redundant initialization of sg_table

sg_table is being initialized and is never read before it is updated
again later on, hence making the initialization redundant. Remove
the initialization.

Detected by clang scan-build:
"warning: Value stored to 'sg_table' during its initialization is
never read"
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170914230516.6056-1-colin.king@canonical.com

drivers/dma-buf/dma-buf.c

@@ -625,7 +625,7 @@ EXPORT_SYMBOL_GPL(dma_buf_detach);
 struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
 					enum dma_data_direction direction)
 {
-	struct sg_table *sg_table = ERR_PTR(-EINVAL);
+	struct sg_table *sg_table;
 
 	might_sleep();