MFC r268505, r268507:

Avoid relocking filedesc lock when closing fds during fdp destruction.

Don't call bzero nor fdunused from fdfree for such cases. It would do
unnecessary work and complain that the lock is not taken.

Don't zero fd_nfiles during fdp destruction.

Code trying to take a look has to check fd_refcnt and it is 0 by that time.

This is a follow up to r268505, without this the code would leak memory for
tables bigger than the default.