fsnotify: add groups to fsnotify_inode_groups when registering inode watch

Currently all fsnotify groups are added immediately to the
fsnotify_inode_groups list upon creation.  This means, even groups with no
watches (common for audit) will be on the global tracking list and will
get checked for every event.  This patch adds groups to the global list on
when the first inode mark is added to the group.

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