test/librbd: correct expected_overlap in SnapshotCopyup

test/librbd: correct expected_overlap in SnapshotCopyup

Changing the end of second interval from 2096640 to copyup_end - 512
with copyup_end potentially set to 1 << order in commit 750e61ac91d7
("librbd: clone copy-on-write operations should preserve sparseness")
was incorrect because the test image size is just 2M.  There are no
end-to-end tests for enable_sparse_copyup = false case, so this went
unnoticed.

A year later, commit 38622b5ca12d ("librbd: copyup state machine
should always issue a sparse-read") dropped the respective branch in
CopyupRequest, thus eliminating the reason for branching on
enable_sparse_copyup altogether.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>