mm: fix build on non-mmu machines

Commit 1ea0704e aka "mm: add a ptep_modify_prot transaction abstraction"

caused:

|  CC      init/main.o
|In file included from include2/asm/pgtable.h:68,
|                 from /home/bigeasy/git/linux-2.6-m68k/include/linux/mm.h:39,
|                 from include2/asm/uaccess.h:8,
|                 from /home/bigeasy/git/linux-2.6-m68k/include/linux/poll.h:13,
|                 from /home/bigeasy/git/linux-2.6-m68k/include/linux/rtc.h:113,
|                 from /home/bigeasy/git/linux-2.6-m68k/include/linux/efi.h:19,
|                 from /home/bigeasy/git/linux-2.6-m68k/init/main.c:43:
|/linux-2.6/include/asm-generic/pgtable.h: In function '__ptep_modify_prot_start':
|/linux-2.6/include/asm-generic/pgtable.h:209: error: implicit declaration of function 'ptep_get_and_clear'
|/linux-2.6/include/asm-generic/pgtable.h:209: error: incompatible types in return
|/linux-2.6/include/asm-generic/pgtable.h: In function '__ptep_modify_prot_commit':
|/linux-2.6/include/asm-generic/pgtable.h:220: error: implicit declaration of function 'set_pte_at'
|make[2]: *** [init/main.o] Error 1
|make[1]: *** [init] Error 2
|make: *** [sub-make] Error 2

on my m68knommu box.
Acked-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: NSebastian Siewior <bigeasy@linutronix.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/pgtable.h

@@ -195,7 +195,6 @@ static inline int pmd_none_or_clear_bad(pmd_t *pmd)
 	}
 	return 0;
 }
-#endif /* CONFIG_MMU */
 
 static inline pte_t __ptep_modify_prot_start(struct mm_struct *mm,
 					     unsigned long addr,
@@ -253,6 +252,7 @@ static inline void ptep_modify_prot_commit(struct mm_struct *mm,
 	__ptep_modify_prot_commit(mm, addr, ptep, pte);
 }
 #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */
+#endif /* CONFIG_MMU */
 
 /*
  * A facility to provide lazy MMU batching.  This allows PTE updates and