libmount: ignore unwanted kernel events in monitor

libmount: ignore unwanted kernel events in monitor

If the mount operation requires multiple steps, it's optimal for the
libmount monitor to ignore the steps until we get a complete mount
(reported by the utab.event file).

This commit adds a new API function, mnt_monitor_veil_kernel(), to
ignore unwanted kernel mount events.

Note that this only makes sense when the application monitors kernel
and userspace events simultaneously.

Signed-off-by: Karel Zak <kzak@redhat.com>