mld_domifattach() does a memory allocation under the global MLD mutex,
and so can fail (but no error handling prevents a null pointer
dereference in this case). However, the mutex is only needed when
updating the global softc list; the allocation and static initialization
of the softc does not require this mutex. So, reduce the scope of the
mutex and use M_WAITOK for the allocation.

No functional change intended.

PR: 261457