osd: do not include logbl in scrub map

This is a potentially use object/file, usually prefixed by a zeroed region
on disk, that is not used by scrub at all.  It dates back to
f51348dc8bdd5071b7baaf3f0e4d2e0496618f08 (2008) and the original version of
scrub.

This *might* fix #4179.  It is not a leak per se, but I observed 1GB
scrub messages going over the write.  Maybe the allocations are causing
fragmentation, or the sub_op queues are growing.

Signed-off-by: Sage Weil <sage@inktank.com>