Use the right _MAX constant.

Though uio_resid is of type ssize_t, we need to take into account that
this source file contains an implementation specific to a certain
userspace pointer size. If this file provided 32-bit implementations,
this should have used INT32_MAX, even when running a 64-bit kernel.

This change has no effect, but is simply in preparation for adding
support for running 32-bit CloudABI executables.