mm: restrict gfp mask in mpage_alloc

Sergey has reported that we might hit BUG_ON in new_slab() because
unrestricted gfp mask used for the readahead purposes contains
incompatible flags (__GFP_HIGHMEM in his case):
[  429.191962] gfp: 2
[  429.192634] ------------[ cut here ]------------
[  429.193281] kernel BUG at mm/slub.c:1616!
[...]
[  429.217369]  [<ffffffff811ca221>] bio_alloc_bioset+0xbd/0x1b1
[  429.218013]  [<ffffffff81148078>] mpage_alloc+0x28/0x7b
[  429.218650]  [<ffffffff8114856a>] do_mpage_readpage+0x43d/0x545
[  429.219282]  [<ffffffff81148767>] mpage_readpages+0xf5/0x152

Make sure that mpage_alloc always restricts the mask to GFP_KERNEL subset.
This is what was done before "mm, memcg: use consistent gfp flags during
readahead" explicitly by mapping_gfp_constraint(mapping, GFP_KERNEL) in
mpage_readpages.

Link: http://lkml.kernel.org/r/20160610074223.GC32285@dhcp22.suse.cz
Reported-by: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/mpage.c

@@ -71,6 +71,8 @@ mpage_alloc(struct block_device *bdev,
 {
 	struct bio *bio;
 
+	/* Restrict the given (page cache) mask for slab allocations */
+	gfp_flags &= GFP_KERNEL;
 	bio = bio_alloc(gfp_flags, nr_vecs);
 
 	if (bio == NULL && (current->flags & PF_MEMALLOC)) {