btrfs: separate superblock items out of fs_info

fs_info has now ~9kb, more than fits into one page. This will cause
mount failure when memory is too fragmented. Top space consumers are
super block structures super_copy and super_for_commit, ~2.8kb each.
Allocate them dynamically. fs_info will be ~3.5kb. (measured on x86_64)

Add a wrapper for freeing fs_info and all of it's dynamically allocated
members.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index f63c9b3..5181c53 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -936,8 +936,8 @@
 	wait_queue_head_t transaction_blocked_wait;
 	wait_queue_head_t async_submit_wait;
 
-	struct btrfs_super_block super_copy;
-	struct btrfs_super_block super_for_commit;
+	struct btrfs_super_block *super_copy;
+	struct btrfs_super_block *super_for_commit;
 	struct block_device *__bdev;
 	struct super_block *sb;
 	struct inode *btree_inode;
@@ -2387,6 +2387,18 @@
 	smp_mb();
 	return fs_info->closing;
 }
+static inline void free_fs_info(struct btrfs_fs_info *fs_info)
+{
+	kfree(fs_info->delayed_root);
+	kfree(fs_info->extent_root);
+	kfree(fs_info->tree_root);
+	kfree(fs_info->chunk_root);
+	kfree(fs_info->dev_root);
+	kfree(fs_info->csum_root);
+	kfree(fs_info->super_copy);
+	kfree(fs_info->super_for_commit);
+	kfree(fs_info);
+}
 
 /* root-item.c */
 int btrfs_find_root_ref(struct btrfs_root *tree_root,