MINOR: buffer: use MT_LIST_ADDQ() for buffer_wait lists additions

MINOR: buffer: use MT_LIST_ADDQ() for buffer_wait lists additions

The TRY_ADDQ there was not needed since the wait list is exclusively
owned by the caller. There's a preliminary test on MT_LIST_ADDED()
that might have been eliminated by keeping MT_LIST_TRY_ADDQ() but
it would have required two more expensive writes before testing so
better keep the test the way it is.