Adapt API style of lock_memory to match the one of the other functions (backport #209)

[mergify]

This PR adapts the API style of the lock_memory function to match the style of the other methods.

I will create a PR that adapts the usage in ros2 control:
https://github.com/search?q=repo%3Aros-controls%2Fros2_control%20lock_memory&type=code

Which is also the only place it is currently used:
https://github.com/search?q=realtime_tools%3A%3Alock_memory&type=code

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This is an automatic backport of pull request #209 done by Mergify.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 76.15%. Comparing base (397209c) to head (7659aed).

Files with missing lines	Patch %	Lines
src/realtime_helpers.cpp	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           humble     #229      +/-   ##
==========================================
- Coverage   77.28%   76.15%   -1.14%     
==========================================
  Files           8        8              
  Lines         471      478       +7     
  Branches      179      183       +4     
==========================================
  Hits          364      364              
- Misses         70       77       +7     
  Partials       37       37              

Flag	Coverage Δ
unittests	76.15% <0.00%> (-1.14%)	⬇️

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Files with missing lines	Coverage Δ
src/realtime_helpers.cpp	44.23% <0.00%> (-3.20%)	⬇️