Basically we don't allow a transaction to get too big, e.g., to
avoid contiguously occupation of CPU.

We break and queue the transaction here whenever we have collected
osd_target_transaction_size temp objects.

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

It is never used and I see no chance it will be used.

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

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>

Hard code transaction size to be 300 is not good. Since we already
introduce osd_target_transaction_size for the same purpose, we can
use it directly here.

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

For a better understanding since it can confuse newcommers.

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