fs: btrfs: Crossport open_ctree_fs_info() from btrfs-progs

open_ctree_fs_info() is the main entry point to open btrfs.

This version is a simplfied version of __open_ctree_fd() of btrfs-progs,
the main differences are:
- Parameters on how to specify a block device
  Instead of @fd and @path, U-Boot uses blk_desc and disk_partition_t.

- Remove open_ctree flags
  There won't be multiple open ctree modes in U-Boot.

Otherwise functions structures are all kept the same.

With open_ctree_fs_info() implemented, also introduce the global
current_fs_info pointer to show the current opened btrfs.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>

fs/btrfs/root-tree.c

@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+#include "ctree.h"
+
+int btrfs_find_last_root(struct btrfs_root *root, u64 objectid,
+			struct btrfs_root_item *item, struct btrfs_key *key)
+{
+	struct btrfs_path *path;
+	struct btrfs_key search_key;
+	struct btrfs_key found_key;
+	struct extent_buffer *l;
+	int ret;
+	int slot;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	search_key.objectid = objectid;
+	search_key.type = BTRFS_ROOT_ITEM_KEY;
+	search_key.offset = (u64)-1;
+
+	ret = btrfs_search_slot(NULL, root, &search_key, path, 0, 0);
+	if (ret < 0)
+		goto out;
+	if (path->slots[0] == 0) {
+		ret = -ENOENT;
+		goto out;
+	}
+
+	BUG_ON(ret == 0);
+	l = path->nodes[0];
+	slot = path->slots[0] - 1;
+	btrfs_item_key_to_cpu(l, &found_key, slot);
+	if (found_key.type != BTRFS_ROOT_ITEM_KEY ||
+	    found_key.objectid != objectid) {
+		ret = -ENOENT;
+		goto out;
+	}
+	read_extent_buffer(l, item, btrfs_item_ptr_offset(l, slot),
+			   sizeof(*item));
+	memcpy(key, &found_key, sizeof(found_key));
+	ret = 0;
+out:
+	btrfs_free_path(path);
+	return ret;
+}