osd: fix rare race for pg relevant events

osd: fix rare race for pg relevant events

Theoretically even if _have_pg() returns ture, we still can't assert that
_lookup_lock_pg() will always succeed. This is because when we switch between
these two methods, we will drop pg_map_lock, and thus may let a pg removal
sneak in, which may eventually cause divergence.

However this is a really rare case, and is less likely to happen in a
production environment. But this pr provided a safer way to achieve
the same goal and is a little faster by eliminating a duplicated search
from the pg_map, which makes it meaningful.

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