[SCSI] sg: fix incorrect page problem

There's a problem where sg is executing a ->nopage operation on a
compound page, it actually calls get_page() on the first page in the
compound rather than the page which is being mapped.  The fix is to
select the correct page by indexing into the compound.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>