Add a new ioctl for changing the read filter (BIOCSETFNR). This is
just like BIOCSETF but it doesn't drop all the packets buffered on
the discriptor and reset the statistics.

Also, when setting the write filter, don't drop packets waiting to
be read or reset the statistics.

PR: 118486
Submitted by: Matthew Luckie <mluckie@cs.waikato.ac.nz>
MFC after: 1 month