powerpc: Fix reschedule bug in KUAP-unlocked user copy

schedule must not be explicitly called while KUAP is unlocked, because
the AMR register will not be saved across the context switch on
64s (preemption is allowed because that is driven by interrupts which do
save the AMR).

exit_vmx_usercopy() runs inside an unlocked user access region, and it
calls preempt_enable() which will call schedule() if need_resched() was
set while non-preemptible. This can cause tasks to run unprotected when
the should not, and can cause the user copy to be improperly blocked
when scheduling back to it.

Fix this by avoiding the explicit resched for preempt kernels by
generating an interrupt to reschedule the context if need_resched() got
set.

Reported-by: Samuel Holland <[email protected]>
Signed-off-by: Nicholas Piggin <[email protected]>
Tested-by: Guenter Roeck <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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