Clean up inode alias tracking

Clean up inode alias tracking

New helper functions:

	- d_associate()
	- d_disassociate()

i_nlink, d_inode, and i_dentry are no longer changed outside of these
functions.  We maintain the invariants:

	i_nlink == list_size(&i_dentry) == # of d_inode references

put_inode() is removed; free_inode() is now private to inode.c.

This fixes at least one bug: when freeing a dentry, it was not being
deleted from the inode's dentry list.