RISC-V: add a spin_shadow_stack declaration

The patchwork automation reported a sparse complaint that
spin_shadow_stack was not declared and should be static:
../arch/riscv/kernel/traps.c:335:15: warning: symbol 'spin_shadow_stack' was not declared. Should it be static?

However, this is used in entry.S and therefore shouldn't be static.
The same applies to the shadow_stack that this pseudo spinlock is
trying to protect, so do like its charge and add a declaration to
thread_info.h
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Fixes: 7e186433 ("riscv: fix race when vmap stack overflow")
Reviewed-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20230210185945.915806-1-conor@kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/thread_info.h

@@ -43,6 +43,7 @@
 #ifndef __ASSEMBLY__
 
 extern long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE / sizeof(long)];
+extern unsigned long spin_shadow_stack;
 
 #include <asm/processor.h>
 #include <asm/csr.h>