generic: test cloning large exents to a file with many small extents

generic: test cloning large exents to a file with many small extents

Test that if we clone a file with some large extents into a file
that has many small extents, when the fs is nearly full, the clone
operation does not fail and produces the correct result.

This is motivated by a bug found in btrfs wich is fixed by the
following patches for the linux kernel:

 [PATCH 1/2] Btrfs: factor out extent dropping code from hole punch handler
 [PATCH 2/2] Btrfs: fix ENOSPC errors, leading to transaction aborts, when
             cloning extents

The test currently passes on xfs.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>