mm: provide a fallback for PAGE_KERNEL_EXEC for architectures

Some architectures just don't have PAGE_KERNEL_EXEC.  The mm/nommu.c and
mm/vmalloc.c code have been using PAGE_KERNEL as a fallback for years.
Move this fallback to asm-generic.

Link: http://lkml.kernel.org/r/20180510185507.2439-3-mcgrof@kernel.org
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/pgtable.h

@@ -1109,6 +1109,10 @@ static inline bool arch_has_pfn_modify_check(void)
 # define PAGE_KERNEL_RO PAGE_KERNEL
 #endif
 
+#ifndef PAGE_KERNEL_EXEC
+# define PAGE_KERNEL_EXEC PAGE_KERNEL
+#endif
+
 #endif /* !__ASSEMBLY__ */
 
 #ifndef io_remap_pfn_range

mm/nommu.c

@@ -364,10 +364,6 @@ void *vzalloc_node(unsigned long size, int node)
 }
 EXPORT_SYMBOL(vzalloc_node);
 
-#ifndef PAGE_KERNEL_EXEC
-# define PAGE_KERNEL_EXEC PAGE_KERNEL
-#endif
-
 /**
  *	vmalloc_exec  -  allocate virtually contiguous, executable memory
  *	@size:		allocation size

mm/vmalloc.c

@@ -1907,10 +1907,6 @@ void *vzalloc_node(unsigned long size, int node)
 }
 EXPORT_SYMBOL(vzalloc_node);
 
-#ifndef PAGE_KERNEL_EXEC
-# define PAGE_KERNEL_EXEC PAGE_KERNEL
-#endif
-
 /**
  *	vmalloc_exec  -  allocate virtually contiguous, executable memory
  *	@size:		allocation size