sheep/lib: fix coredump from dump_btree()

After running ./configure with "--enable-debug", the dog will coredumps when
running test case 016. The reason is that the function dump_btree() could only
be called when the type of sd_inode is B-tree.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Reported-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
CHANGELOG: add description for hyper-volume.

Add description of the command-line and detail for hyper-volume.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>