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 <[email protected]>
Fixes: 7e18643 ("riscv: fix race when vmap stack overflow")
Reviewed-by: Guo Ren <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Cc: [email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: ConchuOD

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>