btrfs/283: skip if we cannot write into one extent

On the zoned mode, the extent size is limited also by
queue/zone_append_max_bytes. This breaks the assumption that the file "foo"
has a single extent and corrupts the test output.

It is difficult to support the case, so let's just skip the test in this
case.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>