book3s64/hash: Add kfence functionality

Now that linear map functionality of debug_pagealloc is made generic,
enable kfence to use this generic infrastructure.

1. Define kfence related linear map variables.
   - u8 *linear_map_kf_hash_slots;
   - unsigned long linear_map_kf_hash_count;
   - DEFINE_RAW_SPINLOCK(linear_map_kf_hash_lock);
2. The linear map size allocated in RMA region is quite small
   (KFENCE_POOL_SIZE >> PAGE_SHIFT) which is 512 bytes by default.
3. kfence pool memory is reserved using memblock_phys_alloc() which has
   can come from anywhere.
   (default 255 objects => ((1+255) * 2) << PAGE_SHIFT = 32MB)
4. The hash slot information for kfence memory gets added in linear map
   in hash_linear_map_add_slot() (which also adds for debug_pagealloc).

Reported-by: Pavithra Prakash <pavrampu@linux.vnet.ibm.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/5c2b61941b344077a2b8654dab46efa0322af3af.1729271995.git.ritesh.list@gmail.com