book3s64/radix: Refactoring common kfence related functions

Both radix and hash on book3s requires to detect if kfence
early init is enabled or not. Hash needs to disable kfence
if early init is not enabled because with kfence the linear map is
mapped using PAGE_SIZE rather than 16M mapping.
We don't support multiple page sizes for slb entry used for kernel
linear map in book3s64.

This patch refactors out the common functions required to detect kfence
early init is enabled or not.

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/f4a787224fbe5bb787158ace579780c0257f6602.1729271995.git.ritesh.list@gmail.com