LU-19106 lod: check QoS data in pool before down_write

LU-19106 lod: check QoS data in pool before down_write

Just like ltd_qos_is_usable() does, define pool_qos_is_usable()
to check whether QoS data in pool is up-to-date and balanced before
expensive qos write lock is taken.

Fixes: e642e75cde02 ("LU-13363 lod: do object allocation in OST pool")
Test-Parameters: ostcount=8 testlist=conf-sanity env=ONLY=133
Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: I9d17108b649ba5689f02d5f5eee098d030db3d5b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59852
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: Shuichi Ihara <sihara@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>