Make sure to always clear p_fd for process getting rid of its filetable.

Filetable can be shared with other processes. Previous code failed to
clear the pointer for all but the last process getting rid of the table.
This is mostly cosmetics.

Get rid of 'This should happen earlier' comment. Clearing the pointer in
this place is fine as consumers can reliably check for files availability
by inspecting fd_refcnt and vnodes availabity by NULL-checking them.

MFC after:	1 week