generic: test premature ENOSPC in zoned garbage collection

This test stresses garbage collection in zoned file systems by
constantly overwriting the same file. It is inspired by a reproducer
for a btrfs bugfix.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Zorro Lang <zlang@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>