snapuserd: typecast cow_op->new_block to uint64_t

On 32-bit devices, cow_op->new_block * BLOCK_SZ can overflow.

Bug: 376978542
Test: snapuserd_test on 32-bit device
1: Initiate merge on partition > 4GB.
2: Verify no overflow.

Change-Id: I4efc24891e0b330505ddf59b7db7716f054cf9c4
Signed-off-by: Akilesh Kailash <akailash@google.com>