Undo the workaround at b17a6d33.

If there are lots of memory operations in a TB, the slow path code
can exceed the highwater reservation.  Add a check within the loop.

Tested-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>