powerpc: Use task_stack_page() in current_pt_regs()

Change current_pt_regs() to use task_stack_page() rather than
current_thread_info() so that it keeps working once we enable
THREAD_INFO_IN_TASK.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Split out of large patch]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>