scsi: sg: sg_write(): get rid of access_ok()/__copy_from_user()/__get_user()

Just use plain copy_from_user() and get_user().  Note that while a
buf-derived pointer gets stored into ->dxferp, all places that actually use
the resulting value feed it either to import_iovec() or to
import_single_range(), and both will do validation.

Link: https://lore.kernel.org/r/20191017193925.25539-7-viro@ZenIV.linux.org.ukSigned-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>

drivers/scsi/sg.c

@@ -612,11 +612,9 @@ sg_write(struct file *filp, const char __user *buf, size_t count, loff_t * ppos)
 	      scsi_block_when_processing_errors(sdp->device)))
 		return -ENXIO;
 
-	if (!access_ok(buf, count))
-		return -EFAULT;	/* protects following copy_from_user()s + get_user()s */
 	if (count < SZ_SG_HEADER)
 		return -EIO;
-	if (__copy_from_user(&old_hdr, buf, SZ_SG_HEADER))
+	if (copy_from_user(&old_hdr, buf, SZ_SG_HEADER))
 		return -EFAULT;
 	blocking = !(filp->f_flags & O_NONBLOCK);
 	if (old_hdr.reply_len < 0)
@@ -626,7 +624,7 @@ sg_write(struct file *filp, const char __user *buf, size_t count, loff_t * ppos)
 		return -EIO;	/* The minimum scsi command length is 6 bytes. */
 
 	buf += SZ_SG_HEADER;
-	if (__get_user(opcode, buf))
+	if (get_user(opcode, buf))
 		return -EFAULT;
 
 	if (!(srp = sg_add_request(sfp))) {
@@ -676,7 +674,7 @@ sg_write(struct file *filp, const char __user *buf, size_t count, loff_t * ppos)
 	hp->flags = input_size;	/* structure abuse ... */
 	hp->pack_id = old_hdr.pack_id;
 	hp->usr_ptr = NULL;
-	if (__copy_from_user(cmnd, buf, cmd_size))
+	if (copy_from_user(cmnd, buf, cmd_size))
 		return -EFAULT;
 	/*
 	 * SG_DXFER_TO_FROM_DEV is functionally equivalent to SG_DXFER_FROM_DEV,