NFSD: Cap the number of bytes copied by nfs4_reset_recoverydir()

It's only current caller already length-checks the string, but let's
be safe.

Fixes: 0964a3d3f1aa ("[PATCH] knfsd: nfsd4 reboot dirname fix")
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/nfsd/nfs4recover.c

···
 		return status;
 	status = -ENOTDIR;
 	if (d_is_dir(path.dentry)) {
-		strcpy(user_recovery_dirname, recdir);
+		strscpy(user_recovery_dirname, recdir,
+			sizeof(user_recovery_dirname));
 		status = 0;
 	}
 	path_put(&path);