Remove iov_iter_advance() for iter_write

It seems like this should have gone with openzfs#11378.

The additional iter advance is incorrect, as copy_from_iter() has
already done the right thing.

Closes: openzfs#12041

Suggested-by: @siebenmann
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>

module/os/linux/zfs/zpl_file.c

@@ -342,9 +342,6 @@ zpl_iter_write(struct kiocb *kiocb, struct iov_iter *from)
 	ssize_t wrote = count - uio.uio_resid;
 	kiocb->ki_pos += wrote;
 
-	if (wrote > 0)
-		iov_iter_advance(from, wrote);
-
 	return (wrote);
 }