riscv/mm: Fix the comment for swap pte format

Swap type takes bits 7-11 and swap offset should start from bit 12.
Signed-off-by: Xiao Wang <xiao.w.wang@intel.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20230921141652.2657054-1-xiao.w.wang@intel.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/pgtable.h

@@ -811,7 +811,7 @@ extern pmd_t pmdp_collapse_flush(struct vm_area_struct *vma,
  *	bit            5:	_PAGE_PROT_NONE (zero)
  *	bit            6:	exclusive marker
  *	bits      7 to 11:	swap type
- *	bits 11 to XLEN-1:	swap offset
+ *	bits 12 to XLEN-1:	swap offset
  */
 #define __SWP_TYPE_SHIFT	7
 #define __SWP_TYPE_BITS		5