LU-14277 lod: statfs should not block a create

LU-14277 lod: statfs should not block a create

lod_qos_statfs_update() need a guarantee that targets
isn't changed, so it doesn't need to take a QoS mutex.

HPE-bug-id: LUS-2106
Signed-off-by: Alexander Boyko <alexander.boyko@hpe.com>
Signed-off-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Change-Id: Id9f0ea2fa006bee601d05e14b2e515fcf8248249
Reviewed-on: https://review.whamcloud.com/41497
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>