Reduce ObjectCacher flush overhead

Flush op in ObjectCacher will iterate the whole active object set, each
dirty object also may own several BufferHead. If the object set is large,
it will consume too much time.

Use dirty_bh instead to reduce overhead. Now only dirty BufferHead will
be checked.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Add rbdcache max dirty object option

Librbd will calculate max dirty object according to rbd_cache_max_size, it
doesn't suitable for every case. If user set image order 24, the calculating
result is too small for reality. It will increase the overhead of trim call
which is called each read/write op.

Now we make it as option for tunning, by default this value is calculated.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>