Fix potential cases where max_dust_htlc_exposure_msat overflows

[benthecarman]

These are the few cases I found that max_dust_htlc_exposure_msat could be overflowing, there could be more

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b3e7aac) 88.75% compared to head (55da9c4) 88.74%.
Report is 35 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2722      +/-   ##
==========================================
- Coverage   88.75%   88.74%   -0.01%     
==========================================
  Files         112      113       +1     
  Lines       88504    89170     +666     
  Branches    88504    89170     +666     
==========================================
+ Hits        78548    79138     +590     
- Misses       7717     7785      +68     
- Partials     2239     2247       +8     

Files	Coverage Δ
lightning/src/ln/channel.rs	88.66% <100.00%> (+0.26%)	⬆️

... and 21 files with indirect coverage changes

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[TheBlueMatt]

Nothing complicated here, updates the only place where FeeRateMultiplier is used in the codebase (outside of tests), and updates all the places where get_max_dust_htlc_exposure_msat returning max_value is problematic. No reason to wait on another reviewer.