LU-18024 quota: relate qmt_lvbo_free_wq and QMT

LU-18024 quota: relate qmt_lvbo_free_wq and QMT

The global variable "qmt_lvbo_free_wq" could be affected if there
are more than one Lustre mount (for instance, using snapshot),
this patch moves it into "struct qmt_device" and it will only be
used by this QMT(MDT0000) and avoid conflict.

Fixes: 2cc18ece1e ("LU-14535 quota: free lvbo in a wq")
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Change-Id: I298382fa0eed6885e55991e26e4b36d435e99e26
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56661
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Sergey Cheremencev <scherementsev@ddn.com>