book3s64/hash: Add hash_debug_pagealloc_alloc_slots() function

This adds hash_debug_pagealloc_alloc_slots() function instead of open
coding that in htab_initialize(). This is required since we will be
separating the kfence functionality to not depend upon debug_pagealloc.

Now that everything required for debug_pagealloc is under a #ifdef
config. Bring in linear_map_hash_slots and linear_map_hash_count
variables under the same config too.

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/d1d5aabe1e4c693a983e59ccf3de08e3c28c5161.1729271995.git.ritesh.list@gmail.com