powerpc: Don't include lppaca.h in paca.h

powerpc: Don't include lppaca.h in paca.h

[ Upstream commit 1aa000667669fa855853decbb1c69e974d8ff716 ]

By adding a forward declaration for struct lppaca we can untangle paca.h
and lppaca.h. Also move get_lppaca() into lppaca.h for consistency.

Add includes of lppaca.h to some files that need it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230823055317.751786-3-mpe@ellerman.id.au
Stable-dep-of: eac030b22ea1 ("powerpc/pseries: Rework lppaca_shared_proc() to avoid DEBUG_PREEMPT")
Signed-off-by: Sasha Levin <sashal@kernel.org>