scsi: sg: sg_new_write(): don't bother with access_ok

scsi: sg: sg_new_write(): don't bother with access_ok

... just use copy_from_user().  We copy only SZ_SG_IO_HDR bytes, so that
would, strictly speaking, loosen the check.  However, for call chains via
->write() the caller has actually checked the entire range and SG_IO passes
exactly SZ_SG_IO_HDR for count.  So no visible behaviour changes happen if
we check only what we really need for copyin.

Link: https://lore.kernel.org/r/20191017193925.25539-5-viro@ZenIV.linux.org.uk
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>