comms/gnuradio: Fix crash with GNU Radio buffers

This patch is a backport of ca44241 from upstream:
runtime: Use MAP_FIXED flag to ensure buffer halves are contiguous

It fixes SIGSEGV observed with GNU Radio buffers consumers such as
comms/gqrx.

Discussed here:

https://github.com/gqrx-sdr/gqrx/issues/1275
https://github.com/gnuradio/gnuradio/pull/6854

PR: 272543
Reported by: trasz
Obtained from: GNU Radio team (GH pull request: 6854)
MFH: 2023Q3