osd/PG.cc: initialize PG::flushed in constructor

Initialize PG::flushed in constructor with false as
described in doc/dev/osd_internals/pg.rst .

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
(cherry picked from commit fb840c8ff75b0c66dfeed48e8558542fe3da4c24)