dma-buf/sync_file: fix documentation error

The ioctl name and description on the documentation block don't
match the ioctl being defined. This was probably overlooked while
renaming the ioctls during the sync file destaging. This patch
provides a more accurate description of what the ioctl actually does.

Signed-off-by: Emilio López <emilio.lopez@collabora.co.uk>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20160919042120.6280-1-emilio.lopez@collabora.co.uk