[SCSI] sg: return EFAULT for an invalid user address

blk_rq_unmap_user() returns EFAULT if a program passes an invalid
address to kernel (the kernel fails to copy data to user space). sg
needs to pass the returned value to user space instead of ignoring
it. Before the block layer conversion, sg returns EFAULT
properly. This restores the old behavior.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>