crimson/os/seastore/transaction_manager: Move CRC_NULL checks

crimson/os/seastore/transaction_manager: Move CRC_NULL checks

check_full_extent_integrity would allow for CRC_NULL (0) checksums
when `full_extent_integrity_check` was false. Instead, move this
check as an assertiion into TransactionManager::read_pin.

This would allow us to reuse CRC_NULL concept for more purposes e.g
skipping integrity checks when no CRC is passed (next commits).

Note: With this change check_full_extent_integrity could be called
      only on non CRC_NULL, the check would be moved in next commits.

Signed-off-by: Matan Breizman <mbreizma@redhat.com>