Revert "Don't read space maps during import for readonly pools"

This reverts commit 7fc8c33ede10f7104ca0e91d690d3ebb5236887b which
accidentally introduced a ztest failure.

ztest: '/usr/sbin/zdb -bcc -d -U /var/tmp/zpool.cache ztest' exit code 2
child exited with code 3

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>