ext4: clean up ext4_ext_convert_to_initialized() error handling

If ext4_ext_convert_to_initialized() fails when called within
ext4_ext_handle_unwritten_extents(), immediately error out through the
exit point at function end.  Fix the error handling in the event
ext4_ext_convert_to_initialized() returns 0, which it shouldn't do when
converting an existing extent.  The current code returns the passed in
value of allocated (which is likely non-zero) while failing to set
m_flags, m_pblk, and m_len.
Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Link: https://lore.kernel.org/r/20200430185320.23001-5-enwlinux@gmail.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/extents.c

@@ -3872,15 +3872,28 @@ ext4_ext_handle_unwritten_extents(handle_t *handle, struct inode *inode,
 		goto out1;
 	}
 
-	/* buffered write, writepage time, convert*/
+	/*
+	 * Default case when (flags & EXT4_GET_BLOCKS_CREATE) == 1.
+	 * For buffered writes, at writepage time, etc.  Convert a
+	 * discovered unwritten extent to written.
+	 */
 	ret = ext4_ext_convert_to_initialized(handle, inode, map, ppath, flags);
-	if (ret >= 0)
-		ext4_update_inode_fsync_trans(handle, inode, 1);
-
-	if (ret <= 0) {
+	if (ret < 0) {
 		err = ret;
 		goto out2;
 	}
+	ext4_update_inode_fsync_trans(handle, inode, 1);
+	/*
+	 * shouldn't get a 0 return when converting an unwritten extent
+	 * unless m_len is 0 (bug) or extent has been corrupted
+	 */
+	if (unlikely(ret == 0)) {
+		EXT4_ERROR_INODE(inode, "unexpected ret == 0, m_len = %u",
+				 map->m_len);
+		err = -EFSCORRUPTED;
+		goto out2;
+	}
+
 out:
 	allocated = ret;
 	map->m_flags |= EXT4_MAP_NEW;