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

module/zfs/dbuf.c

@@ -2921,7 +2921,7 @@ dmu_buf_will_fill(dmu_buf_t *db_fake, dmu_tx_t *tx, boolean_t canfail)
 		 * pending clone and mark the block as uncached. This will be
 		 * as if the clone was never done.
 		 */
-		if (dr && dr->dt.dl.dr_brtwrite) {
+		if (db->db_state == DB_NOFILL) {
 			VERIFY(!dbuf_undirty(db, tx));
 			db->db_state = DB_UNCACHED;
 		}

module/zfs/dmu.c

@@ -1221,6 +1221,7 @@ dmu_read_impl(dnode_t *dn, uint64_t offset, uint64_t size,
 			bufoff = offset - db->db_offset;
 			tocpy = MIN(db->db_size - bufoff, size);
 
+			ASSERT(db->db_data != NULL);
 			(void) memcpy(buf, (char *)db->db_data + bufoff, tocpy);
 
 			offset += tocpy;
@@ -1278,6 +1279,7 @@ dmu_write_impl(dmu_buf_t **dbp, int numbufs, uint64_t offset, uint64_t size,
 		else
 			dmu_buf_will_dirty(db, tx);
 
+		ASSERT(db->db_data != NULL);
 		(void) memcpy((char *)db->db_data + bufoff, buf, tocpy);
 
 		if (tocpy == db->db_size)
@@ -1426,6 +1428,7 @@ dmu_read_uio_dnode(dnode_t *dn, zfs_uio_t *uio, uint64_t size)
 		bufoff = zfs_uio_offset(uio) - db->db_offset;
 		tocpy = MIN(db->db_size - bufoff, size);
 
+		ASSERT(db->db_data != NULL);
 		err = zfs_uio_fault_move((char *)db->db_data + bufoff, tocpy,
 		    UIO_READ, uio);
 
@@ -1550,6 +1553,7 @@ top:
 		else
 			dmu_buf_will_dirty(db, tx);
 
+		ASSERT(db->db_data != NULL);
 		err = zfs_uio_fault_move((char *)db->db_data + bufoff,
 		    tocpy, UIO_WRITE, uio);