btrfs: sysfs: export supported checksums

Export supported checksum algorithms via sysfs in the list of static
features:

  /sys/fs/btrfs/features/supported_checksums

Space spearated list of checksum algorithm names.

Co-developed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>