Add in a delete_inode method for use when the inode is finally unlinked,

Add in a delete_inode method for use when the inode is finally unlinked,
so that we can clear out the objid if necessary.  Since delete_inode
is handled by a callback from the VFS when n_link == 0 (and we don't
want to introduce race conditions by checking this at unlink time), we
replace the client delete_inode method with a filesystem-specific MDS
delete_inode call, and then call the client delete_inode method from
within our MDS delete_inode operation.