output/buffer: Change buffer-size default value

[jlucovsky]

The buffer-size value that controls file output buffering defaults to 8k. To be consistent with previous logic, the default is being changed to 0 (e.g., needed if there are old config files that don't specifically enable the new value).

Link to ticket: https://redmine.openinfosecfoundation.org/issues/3449

Describe changes:

• Ensure default value for buffer-size is 0, even with old config files.

Provide values to any of the below to override the defaults.

• To use an LibHTP, Suricata-Verify or Suricata-Update pull request,
  link to the pull request in the respective _BRANCH variable.
• Leave unused overrides blank or remove.

SV_REPO=
SV_BRANCH=
SU_REPO=
SU_BRANCH=
LIBHTP_REPO=
LIBHTP_BRANCH=

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.72%. Comparing base (2e52e95) to head (9f7abb3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12696      +/-   ##
==========================================
- Coverage   80.72%   80.72%   -0.01%     
==========================================
  Files         936      936              
  Lines      259393   259393              
==========================================
- Hits       209398   209397       -1     
- Misses      49995    49996       +1     

Flag	Coverage Δ
fuzzcorpus	56.97% <100.00%> (-0.01%)	⬇️
livemode	19.41% <100.00%> (-0.01%)	⬇️
pcap	44.22% <100.00%> (+<0.01%)	⬆️
suricata-verify	63.51% <100.00%> (-0.02%)	⬇️
unittests	58.21% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[suricata-qa]

WARNING:

field	baseline	test	%
	SURI_TLPR1_stats_chk
.uptime	637	616	96.7%

Pipeline 24941