mm/hmm: fix header file if/else/endif maze, again

The last fix was still wrong, as we need the inline dummy functions also
for the case that CONFIG_HMM is enabled but CONFIG_HMM_MIRROR is not:

  kernel/fork.o: In function `__mmdrop':
  fork.c:(.text+0x14f6): undefined reference to `hmm_mm_destroy'

This adds back the second copy of the dummy functions, hopefully
this time in the right place.

Link: http://lkml.kernel.org/r/20180404110236.804484-1-arnd@arndb.de
Fixes: 8900d06a277a ("mm/hmm: fix header file if/else/endif maze")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>