In case if proc_pid_make_inode() steps on exiting task we do
iput() and return NULL.  Unfortunately, in that case inode->i_ino
doesn't look like inumber of a per-process inode and we take the
wrong path in proc_delete_inode().  I.e. do dput(PDE(inode)).  Which
is left uninitialized...

	We used to get out with that almost by accident - that code
worked only because we had zeroed out one field of union and that
guaranteed that another field would be NULL.  It worked, but broke
at the first occasion.