Improved error handling for extreme rewinds

Improved error handling for extreme rewinds

The vdev_checkpoint_sm_object(), vdev_obsolete_sm_object(), and
vdev_obsolete_counts_are_precise() functions assume that the
only way a zap_lookup() can fail is if the requested entry is
missing.  While this is the most common cause, it's not the only
cause.  Attemping to access a damaged ZAP will result in other
errors.

The most likely scenario for accessing a damaged ZAP is during
an extreme rewind pool import.  Under these conditions the pool
is expected to contain damaged objects and the import code was
updated to handle this gracefully.  Getting an ECKSUM error from
these ZAPs after the pool in import a far less likely, therefore
the behavior for call paths was not modified.

Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7809
Closes #7921