By function, chunk item verification is more suitable to be done inside
tree-checker.

So move btrfs_check_chunk_valid() to tree-checker.c and export it.

And since it's now moved to tree-checker, also add a better comment for
what this function is doing.
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: NQu Wenruo <wqu@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>