pie/piegen: make -f explicit parameter

One may specify other parameters, but not 'f'.
Piegen should output usage() instead of trying to open "file.o"
as -f is explicit.
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/pie/piegen/main.c

@@ -18,7 +18,7 @@
 #include "piegen.h"
 
 piegen_opt_t opts = {
-	.input_filename		= "file.o",
+	.input_filename		= NULL,
 	.stream_name		= "stream",
 	.prefix_name		= "__",
 	.var_name		= "elf_relocs",
@@ -116,6 +116,9 @@ int main(int argc, char *argv[])
 		}
 	}
 
+	if (!opts.input_filename)
+		goto usage;
+
 	fd = open(opts.input_filename, O_RDONLY);
 	if (fd < 0) {
 		pr_perror("Can't open file %s", opts.input_filename);