Initialise scatter/gather list in sg driver

After turning on DEBUG_SG I hit a fail:

kernel BUG at include/linux/scatterlist.h:50!

	sg_build_indirect
	sg_build_reserve
	sg_open
	chrdev_open
	__dentry_open
	do_filp_open
	do_sys_open

We should initialise the sg list when we allocate it in sg_build_sgat.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>