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>