Set f_file to -1/F_UNUSED when after closing when possible

This will help ensure we don't trash file descriptors that get used later on
in the daemon

Found via internal Coverity scan

MFC after: 2 weeks
Discussed with: cem, ed, markj
Differential Revision: https://reviews.freebsd.org/D3081
Submitted by: Miles Ohlrich <miles.ohlrich@isilon.com>
Sponsored by: EMC / Isilon Storage Division