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

(cherry picked from commit 35f73836)