mm/page_alloc.c: mark bad_range() and meminit_pfn_in_nid() as __maybe_unused

mm/page_alloc.c: mark bad_range() and meminit_pfn_in_nid() as __maybe_unused

The functions are not used in some configurations.  Adding the attribute
fixes the following warnings when building with clang:

  mm/page_alloc.c:409:19: error: function 'bad_range' is not needed and
      will not be emitted [-Werror,-Wunneeded-internal-declaration]

  mm/page_alloc.c:1106:30: error: unused function 'meminit_pfn_in_nid'
      [-Werror,-Wunused-function]

Link: http://lkml.kernel.org/r/20170518182030.165633-1-mka@chromium.org
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>