osd/scrub: pre-prepare the description text of the current state

osd/scrub: pre-prepare the description text of the current state

The existing code accesses the states of the scrub FSM via the
standard "state iterator", which is definitely not thread-safe.
It is also a bit time consuming.

As it happened, new code that tried to create the state string
while not holding the PG lock - crashed the OSD.
This PR changes the relevant lines to allow for safe and fast
access to the state description text.

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