osd: fix rare race while looking for a pg

osd: fix rare race while looking for a pg

When _have_pg() returns true, we drop the pg_map_lock
and pg->lock() simultaneously. So theoretically the
subsequent call to _lookup_lock_pg() can return NULL,
although the chance is rare.

Also, since we are going to call _lookup_lock_pg() anyway,
we can call it directly, thus the _have_pg() method is
totally unnecessary.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>