riscv: pgtable: Fixup comment for KERN_VIRT_SIZE

KERN_VIRT_SIZE is 1/4 of the entries of the page global directory,
not half.

Fixes: f7ae0233 ("riscv: Move KASAN mapping next to the kernel mapping")
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230110080419.931185-1-guoren@kernel.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/pgtable.h

@@ -31,7 +31,7 @@
 #define PTRS_PER_PTE    (PAGE_SIZE / sizeof(pte_t))
 
 /*
- * Half of the kernel address space (half of the entries of the page global
+ * Half of the kernel address space (1/4 of the entries of the page global
  * directory) is for the direct mapping.
  */
 #define KERN_VIRT_SIZE          ((PTRS_PER_PGD / 2 * PGDIR_SIZE) / 2)