Modify sharenfs=on default behavior

While it may sometimes be convenient to export an NFS filesystem with
no_root_squash it should not be the default behavior.  Align the
default behavior with the Linux NFS server defaults.  To restore
the previous behavior use 'zfs set sharenfs="no_root_squash,..."'.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#9397
Closes openzfs#9425

lib/libshare/nfs.c

@@ -387,9 +387,10 @@ get_linux_shareopts(const char *shareopts, char **plinux_opts)
 
 	*plinux_opts = NULL;
 
-	/* default options for Solaris shares */
+	/* no_subtree_check - Default as of nfs-utils v1.1.0 */
 	(void) add_linux_shareopt(plinux_opts, "no_subtree_check", NULL);
-	(void) add_linux_shareopt(plinux_opts, "no_root_squash", NULL);
+
+	/* mountpoint - Restrict exports to ZFS mountpoints */
 	(void) add_linux_shareopt(plinux_opts, "mountpoint", NULL);
 
 	rc = foreach_nfs_shareopt(shareopts, get_linux_shareopts_cb,

man/man8/zfs.8

@@ -1977,7 +1977,7 @@ If the property is set to
 .Sy on ,
 the dataset is shared using the default options:
 .Pp
-.Em sec=sys,rw,crossmnt,no_subtree_check,no_root_squash
+.Em sec=sys,rw,crossmnt,no_subtree_check
 .Pp
 See
 .Xr exports 5