Allocate shared memory instead of only reserving it

[DL6ER]

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Use fallocate() instead of ftruncate() when resizing and/or creating shared memory objects.
This ensures we reserve the requested memory exclusively for ourselves. With ftruncate we only announce we want the memory, however, other processes can still take the memory when the /dev/shm is close to being full.

This has the effect that this PR ensures that FTL does not crash when /dev/shm is getting full.

In addition, even when the shared memory arena is full, there will be a better error message, like

[2020-10-01 00:18:42.812 13052M] Creating shared memory with name "FTL-lock" and size 48 (/dev/shm: 2.0GB used, 2.0GB total)
[2020-10-01 00:18:42.812 13052M] WARNING: More than 90% of /dev/shm is used
[2020-10-01 00:18:42.812 13052M] FATAL: create_shm(): Failed to resize "FTL-lock" (4) to 48: No space left on device (-1)

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/ftl-hangs-when-reaching-out-of-memory/38688/21

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-core-web-v5-2-and-ftl-v5-3-released/40909/1