riscv: prevent stack corruption by reserving task_pt_regs(p) early

Early function calls, such as setup_vm(), relocate_enable_mmu(),
soc_early_init() etc, are free to operate on stack. However,
PT_SIZE_ON_STACK bytes at the head of the kernel stack are purposedly
reserved for the placement of per-task register context pointed by
task_pt_regs(p). Those functions may corrupt task_pt_regs if we overlap
the $sp with it. In fact, we had accidentally corrupted sstatus.VS in some
tests, treating the kernel to save V context before V was actually
allocated, resulting in a kernel panic.

Thus, we should skip PT_SIZE_ON_STACK for $sp before making C function
calls from the top-level assembly.

Co-developed-by: ShihPo Hung <[email protected]>
Signed-off-by: ShihPo Hung <[email protected]>
Co-developed-by: Vincent Chen <[email protected]>
Signed-off-by: Vincent Chen <[email protected]>
Signed-off-by: Greentime Hu <[email protected]>
Signed-off-by: Andy Chiu <[email protected]>
Reviewed-by: Conor Dooley <[email protected]>
Reviewed-by: Heiko Stuebner <[email protected]>
Tested-by: Heiko Stuebner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

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