[PATCH] Fix readv/writev return value

A patch from Janet Morgan <janetmor@us.ibm.com>

If you feed an iovec with a bad address not at the zeroeth segment into
readv or writev, it returns the wrong value.

	iovec 1:  base is 8050b20 len is 64
	iovec 2:  base is ffffffff len is 64
	iovec 3:  base is 8050ba0 len is 64

The writev should return 64 bytes but is returning 128

This is because we've added the new segment's length into `count'
before running access_ok().

The patch changes it to fix that up on the slow path, if access_ok() fails.

mm/filemap.c

@@ -791,6 +791,7 @@ __generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
 		if (seg == 0)
 			return -EFAULT;
 		nr_segs = seg;
+		count -= iv->iov_len;	/* This segment is no good */
 		break;
 	}
 
@@ -1578,6 +1579,7 @@ generic_file_write_nolock(struct file *file, const struct iovec *iov,
 		if (seg == 0)
 			return -EFAULT;
 		nr_segs = seg;
+		ocount -= iv->iov_len;	/* This segment is no good */
 		break;
 	}
 	count = ocount;