Multiple improvements to inode dirtying

Multiple improvements to inode dirtying

- in a few places we didn't called dirty_inode when we should have
  done.
- after inode_{inc,dec}_link_count we don't need to dirty the inode
  because it has already been done.
- handle I_Dirty properly rather than unpinning the block
- allocate NCredits for inodes so that I_Dirty can succeed.