riscv: fix race when vmap stack overflow

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

riscv: fix race when vmap stack overflow

Currently, when detecting vmap stack overflow, riscv firstly switches
to the so called shadow stack, then use this shadow stack to call the
get_overflow_stack() to get the overflow stack. However, there's
a race here if two or more harts use the same shadow stack at the same
time.

To solve this race, we introduce spin_shadow_stack atomic var, which
will be swap between its own address and 0 in atomic way, when the
var is set, it means the shadow_stack is being used; when the var
is cleared, it means the shadow_stack isn't being used.

[Palmer: Add AQ to the swap, and also some comments.]