scatterlist: protect parameters of the sg_table related macros

Add brackets to protect parameters of the recently added sg_table related
macros from side-effects.

Fixes: 709d6d73 ("scatterlist: add generic wrappers for iterating over sgtable objects")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>