btrfs: Test if btrfs hits EDQUOT without trying to reclaim some space

btrfs: Test if btrfs hits EDQUOT without trying to reclaim some space

Commit a514d63882c3 ("btrfs: qgroup: Commit transaction in advance
to reduce early EDQUOT") is no longer forcing transaction commit to
reclaim space, and only commits transaction asynchronously in
advance to address it.

However the criteria used in async transaction commit is not
comprehensive, thus it doesn't reclaim space automatically.

This test case will check the behavior by:
1) Falloc a large padding file
   This file will take 90% of the qgroup limit

2) Sync the fs
   To reflect the qgroup changes

3) Delete the file
   Qgroup won't reclaim the space until transaction committed.

4) Try to write a file
   If kernel not fixed, qgroup will not automatically commit transaction
   to reclaim the freed space and hit EDQUOT.

This bug is going to be fixed by a patch for kernel titled:
"btrfs: qgroup: Make qgroup async transaction commit more aggressive".

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>