LU-12068 tests: add debug for sanity-lfsck test_6b

LU-12068 tests: add debug for sanity-lfsck test_6b

Dump the lfsck_namespace stats file on error to see if it provides
any more information about why the test is failing.

Test-Parameters: trivial testgroup=review-dne-zfs-part-2 testlist=sanity-lfsck
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: If93202101f33f013b9f5ef56022f76c86b3ebbe5
Reviewed-on: https://review.whamcloud.com/34417
Tested-by: Jenkins
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>