kernel - Do not allow destroyed namecache entries to be re-resolved

kernel - Do not allow destroyed namecache entries to be re-resolved

* Do not allow a destroyed namecache entry to be re-resolved, as it
  might resolve as a completely different file, or even resolve as
  a file when it was originally a directory and so forth.

* Fixes inconsistencies in the current-dir fields in proc->p_fd.

* Note that most VFS's can't re-resolve a disconnected directory
  anyway but HAMMER1 actually can, so this was causing a problem
  in tests with HAMMER1 (the same tests that originally crashed
  tmpfs were also crashing HAMMER1, but for a different reason).