os/bluestore: do not extend blobs with unused bitmap

os/bluestore: do not extend blobs with unused bitmap

If we resize the blob we need to adjust the resolution of the
unused bitmap, and that is only possible for some bit patterns.
For now just ignore blobs with unused blocks.

Add an assert in add_tail() so that we don't forget that
add_tail is (probably) where we'd would do that adjustment.

Signed-off-by: Sage Weil <sage@redhat.com>