dma-buf: Improve a size determination in dma_buf_attach()

dma-buf: Improve a size determination in dma_buf_attach()

Replace the specification of a data structure by a pointer dereference
as the parameter for the operator "sizeof" to make the corresponding size
determination a bit safer according to the Linux coding style convention.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>