virtiofsd: Drop CAP_FSETID if client asked for it

virtiofsd: Drop CAP_FSETID if client asked for it

If client requested killing setuid/setgid bits on file being written, drop
CAP_FSETID capability so that setuid/setgid bits are cleared upon write
automatically.

pjdfstest chown/12.t needs this.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
  dgilbert: reworked for libcap-ng
Reviewed-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>