Fix segment size in compressing core dumps

A core segment is bounded in size only by memory size. On 64-bit
architectures this means a segment can be much larger than 4GB.
However, compress_chunk() takes only a u_int, clamping segment size to
4GB-1, resulting in a truncated core. Everything else, including the
compressor internally, uses size_t, so use size_t at the boundary here.

This dates back to the original refactor back in 2015 (r279801 /
aa14e9b7).

MFC after: 1 week
Sponsored by: Juniper Networks, Inc.