riscv: Mark KASAN tmp* page tables variables as static

riscv: Mark KASAN tmp* page tables variables as static

tmp_pg_dir, tmp_p4d and tmp_pud are only used in kasan_init.c so they
should be declared as static.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306282202.bODptiGE-lkp@intel.com/
Fixes: 96f9d4daf745 ("riscv: Rework kasan population functions")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230704074357.233982-1-alexghiti@rivosinc.com
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>