btrfs: test an incremental send scenario with cloning of unaligned extent

btrfs: test an incremental send scenario with cloning of unaligned extent

Test that doing an incremental send with a file that had its size
decreased and became the destination for a clone operation of an extent
with an unaligned end offset that matches the new file size, works
correctly.

This tests a bug fixed by the following kernel patch:

  "btrfs: send: fix invalid clone operation for file that got its size decreased"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>