Let's support __HAVE_ARCH_PTE_SWP_EXCLUSIVE.

On 64bit, steal one bit from the type.  Generic MM currently only uses 5
bits for the type (MAX_SWAPFILES_SHIFT), so the stolen bit is effectively
unused.

On 32bit we're able to locate unused bits.  As the PTE layout for 32 bit
is very confusing, document it a bit better.

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

Link: https://lkml.kernel.org/r/20230113171026.582290-13-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>