virtiofsd: Fix the help message of posix lock

The commit 88fc1079 disabled remote
posix locks by default. But the --help message still says it is enabled
by default. So fix it to output no_posix_lock.

Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
Message-Id: <20201027081558.29904-1-zhangjiachen.jaycee@bytedance.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/helper.c

@@ -161,7 +161,7 @@ void fuse_cmdline_help(void)
            "                               allowed (default: 10)\n"
            "    -o posix_lock|no_posix_lock\n"
            "                               enable/disable remote posix lock\n"
-           "                               default: posix_lock\n"
+           "                               default: no_posix_lock\n"
            "    -o readdirplus|no_readdirplus\n"
            "                               enable/disable readirplus\n"
            "                               default: readdirplus except with "