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).