seastore: support validating device signature for ceph-lvm

seastore: support validating device signature for ceph-lvm

Write the SEASTORE_SUPERBLOCK_SIGN at offset 0 before
encoding the superblock. If the signature is added inside
the block_sm_superblock_t structure (via DENC), it won't be
written at offset 0 because DENC encoding add some metadata.
Therefore prepend the signature to the bufferlist before
encoding the superblock sothat signature always appears at the
start of the written data

Fixes: https://tracker.ceph.com/issues/71954
Signed-off-by: Mohit Agrawal <moagrawa@redhat.com>