ext4: enforce buffer head state assertion in ext4_da_map_blocks

Remove the code that re-initializes a buffer head with an invalid block
number and BH_New and BH_Delay bits when a matching delayed and
unwritten block has been found in the extent status cache. Replace it
with assertions that verify the buffer head already has this state
correctly set.  The current code masked an inline data truncation bug
that left stale entries in the extent status cache.  With this change,
generic/130 can be used to reproduce and detect that bug.

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20210819144927.25163-3-enwlinux@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -1706,13 +1706,16 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
 		}
 
 		/*
-		 * Delayed extent could be allocated by fallocate.
-		 * So we need to check it.
+		 * the buffer head associated with a delayed and not unwritten
+		 * block found in the extent status cache must contain an
+		 * invalid block number and have its BH_New and BH_Delay bits
+		 * set, reflecting the state assigned when the block was
+		 * initially delayed allocated
 		 */
-		if (ext4_es_is_delayed(&es) && !ext4_es_is_unwritten(&es)) {
-			map_bh(bh, inode->i_sb, invalid_block);
-			set_buffer_new(bh);
-			set_buffer_delay(bh);
+		if (ext4_es_is_delonly(&es)) {
+			BUG_ON(bh->b_blocknr != invalid_block);
+			BUG_ON(!buffer_new(bh));
+			BUG_ON(!buffer_delay(bh));
 			return 0;
 		}