powerpc: simplify ppc_save_regs

[ Upstream commit 37195b82 ]

Adjust the pt_regs pointer so the interrupt frame offsets can be used
to save registers.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221127124942.1665522-7-npiggin@gmail.com


Stable-dep-of: b684c09f ("powerpc: update ppc_save_regs to save current r1 in pt_regs")
Signed-off-by: Sasha Levin <sashal@kernel.org>