dma-buf/sw_sync: fix document of sw_sync_create_fence_data

The structure should really be sw_sync_create_fence_data rather than
sw_sync_ioctl_create_fence which is the function name.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1515988079-8677-1-git-send-email-shawn.guo@linaro.org