riscv: Add pte bit to distinguish swap from invalid

riscv: Add pte bit to distinguish swap from invalid

Previously, invalid PTEs and swap PTEs had the same binary
representation, causing errors when attempting to unmap PROT_NONE
mappings, including implicit unmap on exit.

Typical error:

swap_info_get: Bad swap file entry 40000000007a9879
BUG: Bad page map in process a.out  pte:3d4c3cc0 pmd:3e521401

Cc: [email protected]
Signed-off-by: Stefan O'Rear <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Palmer Dabbelt <[email protected]>