Fix large dnode send stream flag conflict

Fix large dnode send stream flag conflict

Bit 21 of the send stream flags was inadvertently used for two
different features under concurrent development.  To avoid any
future compatibility problems the large dnode flag is being
switched to bit 23 which is unused.

The large dnode feature has only been present in pre-releases of
ZoL and dnodesize defaults to legacy which is compatible with
existing OpenZFS implementations.  Users with dnodesize=auto
needing to use zfs send/recv must update ZoL on both the
source and destination systems.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6139