generic/427 creates a 256 MB filesystem and then writes a 200 MB file,
which fails on Btrfs if mixed mode is not enabled. Raise the threshold
to 1GB, which is where we typically recommend mixed mode.
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>