mm, fs: introduce mapping_gfp_constraint()

There are many places which use mapping_gfp_mask to restrict a more
generic gfp mask which would be used for allocations which are not
directly related to the page cache but they are performed in the same
context.

Let's introduce a helper function which makes the restriction explicit and
easier to track.  This patch doesn't introduce any functional changes.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Michal Hocko <mhocko@suse.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 3c2d4ab..1d47d2e 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -491,7 +491,7 @@
 		 * __GFP_DMA32 to be set in mapping_gfp_mask(inode->i_mapping)
 		 * so shmem can relocate pages during swapin if required.
 		 */
-		BUG_ON((mapping_gfp_mask(mapping) & __GFP_DMA32) &&
+		BUG_ON(mapping_gfp_constraint(mapping, __GFP_DMA32) &&
 				(page_to_pfn(p) >= 0x00100000UL));
 	}