DMU_BACKUP_FEATURE: indicate that bit 28 and 29 are reserved

Bit 28 is used by an internal Nutanix feature which might be
upstreamed in the future.

Bit 29 is the last unused bit. It is reserved to indicate a
to-be-designed extension to the stream format which will accomodate
more feature flags.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Christian Schwarz <christian.schwarz@nutanix.com>
Issue #13795
Closes #13796