geom: create an API to allocate events, and use that storage to send them

g_alloc_event will allocate storage for an opaque event. g_post_event_ep
can use memory returned by g_alloc_event to send an event from a context
that might not be able to allocate the event. Occasionally, we can
alloate memory when we create an object, but not while we're destroy
it. This allows one to allocate at creation time memory to use when
destorying the object.

Reviewed by: jhb
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30544