Allow zfs send to exclude datasets

Add support for a -exclude/-X option to `zfs send` to allow dataset 
hierarchies to be excluded.

Snapshots can be excluded using a channel program; however,
this can result in failures with 'zfs send -R'; this option allows 
them to be excluded.  Fortunately, this required a change only to 
cmd/zfs/zfs_main.c, using the already-existing callback argument 
to zfs_send() that is currently unused.

Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Christian Schwarz <christian.schwarz@nutanix.com>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Sean Eric Fagan <kithrup@mac.com>
Signed-off-by: Sean Eric Fagan <kithrup@mac.com>
Closes #13158

man/man8/zfs-send.8

@@ -40,6 +40,7 @@
 .Nm zfs
 .Cm send
 .Op Fl DLPRbcehnpsvw
+.Op Fl X Ar dataset Ns Oo , Ns Ar dataset Oc Ns ...
 .Op Oo Fl I Ns | Ns Fl i Oc Ar snapshot
 .Ar snapshot
 .Nm zfs
@@ -73,6 +74,7 @@
 .Nm zfs
 .Cm send
 .Op Fl DLPRbcehnpvw
+.Op Fl X Ar dataset Ns Oo , Ns Ar dataset Oc Ns ...
 .Op Oo Fl I Ns | Ns Fl i Oc Ar snapshot
 .Ar snapshot
 .Xc
@@ -140,6 +142,23 @@ If the
 flag is used to send encrypted datasets, then
 .Fl w
 must also be specified.
+.It Fl X , -exclude Ar dataset Ns Oo , Ns Ar dataset Oc Ns ...
+When the
+.Fl R
+flag is given,
+.Fl X
+can be used to specify a list of datasets to be excluded from the
+data stream.
+The
+.Fl X
+option can be used multiple times, or the list of datasets can be
+specified as a comma-separated list, or both.
+.Ar dataset
+must not be the pool's root dataset, and all descendant datasets of
+.Ar dataset
+will be excluded from the send stream.
+Requires
+.Fl R .
 .It Fl e , -embed
 Generate a more compact stream by using
 .Sy WRITE_EMBEDDED