riscv: ftrace: Fixup panic by disabling preemption

In the Linux kernel, the following vulnerability has been resolved:

riscv: ftrace: Fixup panic by disabling preemption

In RISCV, we must use an AUIPC + JALR pair to encode an immediate,

forming a jump that jumps to an address over 4K. This may cause errors

if we want to enable kernel preemption and remove dependency from

patching code with stop_machine(). For example, if a task was switched

out on auipc. And, if we changed the ftrace function before it was

switched back, then it would jump to an address that has updated 11:0

bits mixing with previous XLEN:12 part.

p: patched area performed by dynamic ftrace

ftrace_prologue:

p| REG_S ra, -SZREG(sp)

p| auipc ra, 0x? ------------> preempted

...

change ftrace function

...

p| jalr -?(ra) <------------- switched back

p| REG_L ra, -SZREG(sp)

func:

xxx

ret