+ * Functionally, this is like unmap_underlying_metadata() for a range of
+ * blocks. It is implemented to be more efficient for larger ranges of blocks
+ * though.
+ */
+void unmap_underlying_metadata_ext(struct block_device *bdev, sector_t block,
+				   sector_t len)

Please explain what it does and why you'd call it.  And while we're
naming I think the 'metadata' part is highly confusing.  What it does
is to clear buffers from the block device mapping, nothing about
metadata really.

So how about unmap_buffers_range or similar?