Fix regression in dmu_buf_will_fill()

Fix regression in dmu_buf_will_fill()

Direct I/O implementation added condition to call dbuf_undirty()
only in case of block cloning.  But the condition is not right if
the block is no longer dirty in this TXG, but still in DB_NOFILL
state.  It resulted in block not reverting to DB_UNCACHED and
following NULL de-reference on attempt to access absent db_data.

While there, add assertions for db_data to make debugging easier.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #16829