btrfs: avoid repetitive define BTRFS_FEATURE_INCOMPAT_SUPP

BTRFS_FEATURE_INCOMPAT_SUPP is defined twice, once under
CONFIG_BTRFS_DEBUG and once without it, resulting in repetitive code. The
reason for this is to add experimental features under CONFIG_BTRFS_DEBUG.

To avoid repetitive code, add a common list BTRFS_FEATURE_INCOMPAT_SUPP_STABLE,
and append experimental features only under CONFIG_BTRFS_DEBUG.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>