8265 Reserve send stream flag for large dnode feature

illumos/illumos-gate@bc83969fdbd1cb0d97ba00218c0a3de5c89fba92
https://github.com/illumos/illumos-gate/commit/bc83969fdbd1cb0d97ba00218c0a3de5c89fba92

https://www.illumos.org/issues/8265

Reserve bit 23 in the zfs send stream flags for the large
dnode feature which has been implemented for Linux.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Brian Behlendorf <behlendorf1@llnl.gov>