Revert "util: add find_zero_blocks()"

This reverts commit 6169a0da580295837319d90dfcda3019429dbc8f.
Revert "add sha1_from_buffer()"

This reverts commit 3fbb497c572ef5339d5cbb1c1ff93b52b25e419b.
cache: correct block size calculation for inode object

inode and data object have different sizes and thus cache block size is
different. Current code fail to get the right size for inode object when
its last bit of bmap is set with the fixed block size.

This patch tries to get the dynamic block size based on the oid.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>