osd/scrub: clean-up scrub-store IDs generation

osd/scrub: clean-up scrub-store IDs generation

The Scrub Store IDs (used to identify OMAP entries used for storing
object-specific error data) are generated by attaching some prefix to
the text representation of virtual (invented) hobjects.

This commit cleans up the code by moving the name generation
process (creating temporary un-needed objects, then
extracting the to_str() representation of their identity)
into separate functions.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>