Add zfs_scan_ignore_errors tunable

When it's set, a DTL range will be cleared even if its scan/scrub had
errors. This allows to work around resilver/scrub upon import when the
pool has errors.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #7293