btrfs: remove redundant root argument from btrfs_update_inode_fallback()

The root argument for btrfs_update_inode_fallback() always matches the
root of the given inode, so remove the root argument and get it from the
inode argument.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>