btrfs: sysfs: advertise experimental features only if CONFIG_BTRFS_EXPERIMENTAL=y

We are advertising experimental features through sysfs if
CONFIG_BTRFS_DEBUG is set, without looking if CONFIG_BTRFS_EXPERIMENTAL
is set. This is wrong as it will result in reporting experimental
features as supported when CONFIG_BTRFS_EXPERIMENTAL is not set but
CONFIG_BTRFS_DEBUG is set.

Fix this by checking for CONFIG_BTRFS_EXPERIMENTAL instead of
CONFIG_BTRFS_DEBUG.

Fixes: 67cd3f22 ("btrfs: split out CONFIG_BTRFS_EXPERIMENTAL from CONFIG_BTRFS_DEBUG")
Reviewed-by: Neal Gompa <neal@gompa.dev>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>