Indicative inode hash bitmap implementation. Read details.

Indicative inode hash bitmap implementation. Read details.

Known issues:
- To optimize dentfill() on most architectures we don't fstatat() with
  AT_SYMLINK_NOFOLLOW to get the inode number of the symlink itself.
- The current hash size serves the purpose of one filesystem. To serve
  more and have unique bits, we have to add more bits to prefix dev ID
  to the inode.  Memory consumption will be significant.

This will be reverted in next commit.