[stable-1.9] peristent_ptr: add missing offset calculation

[lukaszstolarczuk]

it wasn't added during one of merge's.

ref. 1c37736

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This change is 

[igchor]

Reviewable status: 0 of 1 files reviewed, all discussions resolved

[codecov]

Codecov Report

Merging #1165 (f8767aa) into stable-1.9 (cceee18) will increase coverage by 0.18%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           stable-1.9    #1165      +/-   ##
==============================================
+ Coverage       87.49%   87.67%   +0.18%     
==============================================
  Files              40       40              
  Lines            3847     3847              
==============================================
+ Hits             3366     3373       +7     
+ Misses            481      474       -7     

Flag	Coverage Δ
tests_clang_debug_cpp17	30.21% <ø> (-0.46%)	⬇️
tests_gcc_debug	93.75% <ø> (+0.22%)	⬆️

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Impacted Files	Coverage Δ
include/libpmemobj++/persistent_ptr.hpp	96.32% <ø> (ø)
...ude/libpmemobj++/container/concurrent_hash_map.hpp	86.69% <0.00%> (+0.71%)	⬆️

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