files: handle errors from fcntl()

files.c:159:21: error: comparison of unsigned expression < 0 is always
false [-Werror,-Wtautological-compare]
        if (p->fown.signum < 0) {
            ~~~~~~~~~~~~~~ ^ ~

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>