btrfs: skip tests that fail when autodefrag is enabled

A couple tests, btrfs/004 and btrfs/308, fail if we pass "-o autodefrag"
in MOUNT_OPTIONS, due to changes in extent layouts. So make sure the
tests are skipped if autodefrag is enabled.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <asj@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>