arm64: ensure ERET from kthread is illegal

For consistency, all tasks have a pt_regs reserved at the highest
portion of their task stack. Among other things, this ensures that a
task's SP is always pointing within its stack rather than pointing
immediately past the end.

While it is never legitimate to ERET from a kthread, we take pains to
initialize pt_regs for kthreads as if this were legitimate. As this is
never legitimate, the effects of an erroneous return are rarely tested.

Let's simplify things by initializing a kthread's pt_regs such that an
ERET is caught as an illegal exception return, and removing the explicit
initialization of other exception context. Note that as
spectre_v4_enable_task_mitigation() only manipulates the PSTATE within
the unused regs this is safe to remove.

As user tasks will have their exception context initialized via
start_thread() or start_compat_thread(), this should only impact cases
where something has gone very wrong and we'd like that to be clearly
indicated.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20201113124937.20574-2-mark.rutland@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>