usnic: properly destroy embedded small send frag

Without this, an `--enable-debug` build would hit an assertion in the
list code when run under valgrind with `--malloc-fill=0xff` or any other
case where malloc returned non-zeroed buffers.

Also allow the normal OBJ_ machinery to handle the constructor
invocation ordering for us instead of doing it by hand (which could have
led to future bugs).

Reviewed-by: jsquyres@cisco.com

cmr=v1.7.4

Depends on trunk functionality in r29095 and r29096.  Refs #3740,#3741.