btrfs: add helper for fs_info->closing

wrap checking of filesystem 'closing' flag and fix a few missing memory
barriers.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 982b5ea..fa4ef18 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -129,7 +129,7 @@
 	if (!btrfs_test_opt(root, AUTO_DEFRAG))
 		return 0;
 
-	if (root->fs_info->closing)
+	if (btrfs_fs_closing(root->fs_info))
 		return 0;
 
 	if (BTRFS_I(inode)->in_defrag)
@@ -229,7 +229,7 @@
 		first_ino = defrag->ino + 1;
 		rb_erase(&defrag->rb_node, &fs_info->defrag_inodes);
 
-		if (fs_info->closing)
+		if (btrfs_fs_closing(fs_info))
 			goto next_free;
 
 		spin_unlock(&fs_info->defrag_inodes_lock);