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/mm/filemap.c b/mm/filemap.c
index 6ef3674..1bb00762 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1722,7 +1722,7 @@
 			goto out;
 		}
 		error = add_to_page_cache_lru(page, mapping, index,
-					GFP_KERNEL & mapping_gfp_mask(mapping));
+				mapping_gfp_constraint(mapping, GFP_KERNEL));
 		if (error) {
 			page_cache_release(page);
 			if (error == -EEXIST) {
@@ -1824,7 +1824,7 @@
 			return -ENOMEM;
 
 		ret = add_to_page_cache_lru(page, mapping, offset,
-				GFP_KERNEL & mapping_gfp_mask(mapping));
+				mapping_gfp_constraint(mapping, GFP_KERNEL));
 		if (ret == 0)
 			ret = mapping->a_ops->readpage(file, page);
 		else if (ret == -EEXIST)