mm: stub out all of swapops.h for !CONFIG_MMU

The whole header file deals with swap entries and PTEs, none of which
can exist for nommu builds.  The current nommu ports have lots of stubs
to allow the inline functions in swapops.h to compile, but as none of
this functionality is actually used there is no point in even providing
it.  This way we don't have to provide the stubs for the upcoming RISC-V
nommu port, and can eventually remove it from the existing ports.

Link: http://lkml.kernel.org/r/20190703122359.18200-4-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/swapops.h

@@ -6,6 +6,8 @@
 #include <linux/bug.h>
 #include <linux/mm_types.h>
 
+#ifdef CONFIG_MMU
+
 /*
  * swapcache pages are stored in the swapper_space radix tree.  We want to
  * get good packing density in that tree, so the index should be dense in
@@ -50,13 +52,11 @@ static inline pgoff_t swp_offset(swp_entry_t entry)
 	return entry.val & SWP_OFFSET_MASK;
 }
 
-#ifdef CONFIG_MMU
 /* check whether a pte points to a swap entry */
 static inline int is_swap_pte(pte_t pte)
 {
 	return !pte_none(pte) && !pte_present(pte);
 }
-#endif
 
 /*
  * Convert the arch-dependent pte representation of a swp_entry_t into an
@@ -360,4 +360,5 @@ static inline int non_swap_entry(swp_entry_t entry)
 }
 #endif
 
+#endif /* CONFIG_MMU */
 #endif /* _LINUX_SWAPOPS_H */