os/bluestore: use default value for non-decoded field in test instances

os/bluestore: use default value for non-decoded field in test instances

The sbid field added in commit a7f8e231 is set during construction
but not persisted to disk. Including it in dump() output causes
discrepancies between original and re-encoded instances, leading
to test failures in readable.sh and check-generated.sh.

Current tests pass because they reuse the same instance for
re-encoding, preserving non-persisted fields. An upcoming change
will allocate fresh instances for each decode operation, which
will expose this issue and break tests.

Use default value for sbid when generating test instances to ensure
consistency between encoding and decoding operations.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>