Revert "mm: fail GFP_DMA allocations when ZONE_DMA is not configured"

This reverts commit a197b59a.

As rmk says:
 "Commit a197b59a (mm: fail GFP_DMA allocations when ZONE_DMA is not
  configured) is causing regressions on ARM with various drivers which
  use GFP_DMA.

  The behaviour up until now has been to silently ignore that flag when
  CONFIG_ZONE_DMA is not enabled, and to allocate from the normal zone.
  However, as a result of the above commit, such allocations now fail
  which causes drivers to fail.  These are regressions compared to the
  previous kernel version."

so just revert it.
Requested-by: Russell King <linux@arm.linux.org.uk>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -2247,10 +2247,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
 
 	if (should_fail_alloc_page(gfp_mask, order))
 		return NULL;
-#ifndef CONFIG_ZONE_DMA
-	if (WARN_ON_ONCE(gfp_mask & __GFP_DMA))
-		return NULL;
-#endif
 
 	/*
 	 * Check the zones suitable for the gfp_mask contain at least one