SCSI: sg: Free data buffers after calling blk_rq_unmap_user

SCSI: sg: Free data buffers after calling blk_rq_unmap_user

commit e27168f8c337b12b8aa8d59c3123c79d2f83603d upstream.

Running sg_luns on s390x with CONFIG_DEBUG_PAGEALLOC enabled fails
with EFAULT from the SG_IO ioctl. The EFAULT is the result from
copy_to_user failing in this call chain:

sg_ioctl
sg_new_read
sg_finish_rem_req
blk_rq_unmap_user
__blk_rq_unmap_user
bio_uncopy_user
__bio_copy_iov
copy_to_user

The sg driver calls sg_remove_scat to free the memory pages before
calling blk_rq_unmap_user that tries to copy the data back to
userspace. Change the order to first call blk_rq_unmap_user before
freeing the pages in sg_remove_scat.

Acked-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>