mm: fix build on non-mmu machines

Commit 1ea0704e0d 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: Jeremy 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: Sebastian Siewior <bigeasy@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>