fsnotify: ignored_mask - excluding notification

The ignored_mask is a new mask which is part of fsnotify marks.  A group's
should_send_event() function can use the ignored mask to determine that
certain events are not of interest.  In particular if a group registers a
mask including FS_OPEN on a vfsmount they could add FS_OPEN to the
ignored_mask for individual inodes and not send open events for those
inodes.

Signed-off-by: Eric Paris <eparis@redhat.com>