Btrfs: fix wrong mapping flags for free space inode

We were passing a flags value that differed from the intention in commit
2b108268 ("Btrfs: don't use highmem for free space cache pages").

This caused problems in a ARM machine, leaving btrfs unusable there.
Reported-by: NMerlijn Wajer <merlijn@wizzup.org>
Tested-by: NMerlijn Wajer <merlijn@wizzup.org>
Signed-off-by: NFilipe Manana <fdmanana@suse.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/free-space-cache.c

@@ -86,7 +86,7 @@ static struct inode *__lookup_free_space_inode(struct btrfs_root *root,
 
 	mapping_set_gfp_mask(inode->i_mapping,
 			mapping_gfp_mask(inode->i_mapping) &
-			~(GFP_NOFS & ~__GFP_HIGHMEM));
+			~(__GFP_FS | __GFP_HIGHMEM));
 
 	return inode;
 }