Change the signature of dynahash's alloc function

Change the signature of dynahash's alloc function

Instead of passing the current memory context to the alloc function
via a shared variable, pass it directly as an argument.

Reviewed-by: Tomas Vondra <tomas@vondra.me>
Discussion: https://www.postgresql.org/message-id/01ab1d41-3eda-4705-8bbd-af898f5007f1@iki.fi