riscv/mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE

Let's support __HAVE_ARCH_PTE_SWP_EXCLUSIVE by stealing one bit from the
offset.  This reduces the maximum swap space per file: on 32bit to 16 GiB
(was 32 GiB).

Note that this bit does not conflict with swap PMDs and could also be used
in swap PMD context later.

While at it, mask the type in __swp_entry().

Link: https://lkml.kernel.org/r/20230113171026.582290-20-david@redhat.com


Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>