powerpc: bad_page_fault get registers from regs

Similar to the previous patch this makes interrupt handler function
types more regular so they can be wrapped with the next patch.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210130130852.2952424-12-npiggin@gmail.com