This is a new ioctl, which allows to request next descriptor
allocated by inotify_add_watch().

https://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs.git/commit/?h=for_next&id=e1603b6effe177210701d3d7132d1b68e7bd2c93

The patch checks this cmd is supported by kernel.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>