In the procedure of recover AGI unlinked lists, if something bad
happenes on one of the unlinked inode in the bucket list, we would call
xlog_recover_clear_agi_bucket() to clear the whole unlinked bucket list,
not the unlinked inodes after the bad one. If we have already added some
inodes to the gc workqueue before the bad inode in the list, we could
get below error when freeing those inodes, and finaly fail to complete
the log recover procedure.

 XFS (ram0): Internal error xfs_iunlink_remove at line 2456 of file
 fs/xfs/xfs_inode.c.  Caller xfs_ifree+0xb0/0x360 [xfs]

The problem is xlog_recover_clear_agi_bucket() clear the bucket list, so
the gc worker fail to check the agino in xfs_verify_agino(). Fix this by
flush workqueue before clearing the bucket.

Fixes: ab23a776 ("xfs: per-cpu deferred inode inactivation queues")
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>