Linux: Adjust gedents64 buffer size to int range [BZ #24740]

The kernel interface uses type unsigned int, but there is an
internal conversion to int, so INT_MAX is the correct limit.
Part of the buffer will always be unused, but this is not a
problem.  Such huge buffers do not occur in practice anyway.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>