btrfs: use struct btrfs_inode inside btrfs_get_name()

Use a struct btrfs_inode in btrfs_get_name() as it's an internal
helper, allowing to remove some use of BTRFS_I.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>