LU-12702 quota: wait pending write before acquiring remotely

LU-12702 quota: wait pending write before acquiring remotely

There is a window that ZFS has updated usage, but
pending_write has not been reduced. This could cause
us to grant more space.

With soft least qunit introduced, after timer reach,
local slave might still have some space to consume,
it is possible that we might exceed this limit.

Test-Parameters: fstype=zfs testlist=sanity-quota,sanity-quota,sanity-quota
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I0e1a94fd0b637e154642de5922a0670780c56ef2
Reviewed-on: https://review.whamcloud.com/41023
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>