os/bluestore: fix OnodeSizeTracking testing

os/bluestore: fix OnodeSizeTracking testing

[  FAILED  ] 1 test, listed below:
[  FAILED  ] ObjectStore/StoreTest.OnodeSizeTracking/2, where GetParam() = "bluestore"

 1 FAILED TEST

The above test failure happens as the bluestore mount() process
will try to load all collections and put them into the coll_map,
which will be also tracked as mempool::bluestore_meta_other.
So total_bytes from mempool won't be equal to zero.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>
(cherry picked from commit 1d97862f716ce24bfe227b192ecc5e1eb3f76757)