Hold required lock when initializing shmem

[AndersAstrand]

According to the documentation, each backend is supposed to hold AddinShmemInitLock when calling ShmemInitStruct. We only did that for half of our calls before this patch.

This doesn't fix any of our current test failures, but was something I noticed while looking through the code.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.50%. Comparing base (167aef2) to head (1dcf3ee).

❌ Your project status has failed because the head coverage (82.50%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@                Coverage Diff                 @@
##           TDE_REL_17_STABLE     #541   +/-   ##
==================================================
  Coverage              82.50%   82.50%           
==================================================
  Files                     25       25           
  Lines                   3207     3207           
  Branches                 508      508           
==================================================
  Hits                    2646     2646           
  Misses                   452      452           
  Partials                 109      109           

Components	Coverage Δ
access	84.91% <ø> (ø)
catalog	87.65% <ø> (-0.03%)	⬇️
common	77.77% <ø> (ø)
encryption	72.97% <ø> (ø)
keyring	73.21% <ø> (ø)
src	94.18% <100.00%> (+0.03%)	⬆️
smgr	96.53% <ø> (ø)
transam	∅ <ø> (∅)

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