Create_cow: Hash of target blocks should not be stored.

Target block hash was in-correctly getting added to map thereby
overriding the source-hash.

Bug: 299011882
Test: Flash Pixel 6 Pro from A->B and from B->A
Change-Id: Ib3887c29cd6b8f2abd50e932273a5cfc2a096bd5
Signed-off-by: Akilesh Kailash <akailash@google.com>