Skip FREEOBJECTS for objects which can't exist

Skip FREEOBJECTS for objects which can't exist

When sending an incremental stream based on a snapshot, the receiving
side must have the same base snapshot.  Thus we do not need to send
FREEOBJECTS records for any objects past the maximum one which exists
locally.

This allows us to send incremental streams (again) to older ZFS
implementations (e.g. ZoL < 0.7) which actually try to free all objects
in a FREEOBJECTS record, instead of bailing out early.

Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Closes #5699
Closes #6507
Closes #6616