btrfs: test if rename handles dir item collision correctly

btrfs: test if rename handles dir item collision correctly

This is a regression test for the issue fixed by the kernel commit titled
"btrfs: correctly calculate item size used when item key collision happens"

In this case, we'll simply rename many forged filename that cause collision
under a directory to see if rename failed and filesystem is forced readonly.

Signed-off-by: ethanwu <ethanwu@synology.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>