sparc/mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE on 32bit

Let's support __HAVE_ARCH_PTE_SWP_EXCLUSIVE by reusing the SRMMU_DIRTY bit
as that seems to be safe to reuse inside a swap PTE.  This avoids having
to steal one bit from the swap offset.

While at it, relocate the swap PTE layout documentation and use the same
style now used for most other archs.  Note that the old documentation was
wrong: we use 20 bit for the offset and the reserved bits were 8 instead
of 7 bits in the ascii art.

Link: https://lkml.kernel.org/r/20230113171026.582290-22-david@redhat.com


Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>