fsnotify: use kmem_cache_zalloc to simplify event initialization

fsnotify event initialization is done entry by entry with almost everything
set to either 0 or NULL.  Use kmem_cache_zalloc and only initialize things
that need non-zero initialization.  Also means we don't have to change
initialization entries based on the config options.

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