hashtable: fix dismissHashtable madvise size

[charsyam]

The bug was in dismissHashtable(), which computes the size passed to zmadvise_dontneed() for the top-level hashtable
tables.

ht->tables[i] points to a contiguous array of bucket objects, but the code used sizeof(bucket *) instead of
sizeof(bucket) when calculating the length. That means it treated the allocation like an array of pointers rather than an
array of buckets.

As a result, the advised range was much smaller than the actual table allocation. On 64-bit builds, bucket is 64 bytes
while bucket * is 8 bytes, so only about one eighth of the table was covered. This does not usually break correctness,
but it defeats the purpose of the function: after a fork, we want to tell the kernel that the hashtable pages are no
longer needed so we reduce copy-on-write overhead. With the wrong size, most of the table memory was never included in
that hint.

The fix is to use sizeof(bucket) so the full top-level bucket array is passed to zmadvise_dontneed().

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.45%. Comparing base (4a42c95) to head (81012b3).

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #3533      +/-   ##
============================================
+ Coverage     76.40%   76.45%   +0.05%     
============================================
  Files           159      159              
  Lines         79851    79851              
============================================
+ Hits          61008    61049      +41     
+ Misses        18843    18802      -41     

Files with missing lines	Coverage Δ
src/hashtable.c	97.84% <100.00%> (ø)

... and 18 files with indirect coverage changes

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