virtiofsd: Add docs/helper for killpriv_v2/no_killpriv_v2 option

virtiofsd has introduced killpriv_v2/no_killpriv_v2 for a while. Add
description of it to docs/helper.

Signed-off-by: Liu Yiding <liuyd.fnst@fujitsu.com>
Message-Id: <20220421095151.2231099-1-liuyd.fnst@fujitsu.com>

[Small documentation fixes: s/as client supports/as the client supports/
and s/.  /. /.
--Stefan]

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

docs/tools/virtiofsd.rst

@@ -111,6 +111,11 @@ Options
     label. Server will try to set that label on newly created file
     atomically wherever possible.
 
+  * killpriv_v2|no_killpriv_v2 -
+    Enable/disable ``FUSE_HANDLE_KILLPRIV_V2`` support. KILLPRIV_V2 is enabled
+    by default as long as the client supports it. Enabling this option helps
+    with performance in write path.
+
 .. option:: --socket-path=PATH
 
   Listen on vhost-user UNIX domain socket at PATH.

tools/virtiofsd/helper.c

@@ -188,6 +188,9 @@ void fuse_cmdline_help(void)
            "    -o announce_submounts      Announce sub-mount points to the guest\n"
            "    -o posix_acl/no_posix_acl  Enable/Disable posix_acl. (default: disabled)\n"
            "    -o security_label/no_security_label  Enable/Disable security label. (default: disabled)\n"
+           "    -o killpriv_v2/no_killpriv_v2\n"
+           "                               Enable/Disable FUSE_HANDLE_KILLPRIV_V2.\n"
+           "                               (default: enabled as long as client supports it)\n"
            );
 }