Btrfs: use find_or_create_page instead of grab_cache_page

grab_cache_page will use mapping_gfp_mask(), which for all inodes is set to
GFP_HIGHUSER_MOVABLE.  So instead use find_or_create_page in all cases where we
need GFP_NOFS so we don't deadlock.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index fd7fa2a..6377713 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -307,7 +307,7 @@
 		if (!num_entries && !num_bitmaps)
 			break;
 
-		page = grab_cache_page(inode->i_mapping, index);
+		page = find_or_create_page(inode->i_mapping, index, GFP_NOFS);
 		if (!page)
 			goto free_cache;
 
@@ -594,7 +594,7 @@
 	 * know and don't freak out.
 	 */
 	while (index < num_pages) {
-		page = grab_cache_page(inode->i_mapping, index);
+		page = find_or_create_page(inode->i_mapping, index, GFP_NOFS);
 		if (!page) {
 			int i;