hashmap: make sure to initialize shared hash key atomically

if we allocate a bunch of hash tables all at the same time, with none
earlier than the other, there's a good chance we'll initialize the
shared hash key multiple times, so that some threads will see a
different shared hash key than others.

Let's fix that, and make sure really everyone sees the same hash key.

Fixes: #17007
(cherry picked from commit ae0b700a)
(cherry picked from commit e662cf6d)
(cherry picked from commit 1557cfcf)
(cherry picked from commit bda316cc)