[functional-tests] Move get-dev-size to C code

Saves forking blockdev

ft-lib/dm-ioctl.c

@@ -1,5 +1,6 @@
 #include <linux/dm-ioctl.h>
 #include <linux/kdev_t.h>
+#include <linux/fs.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdlib.h>
@@ -596,4 +597,18 @@ int dm_message(struct dm_interface *dmi, const char *name, uint64_t sector,
 	return r;
 }
 
+int get_dev_size(const char *path, uint64_t *sectors)
+{
+	int r, fd;
+
+	fd = open(path, O_RDONLY);
+	if (fd < 0)
+		return -EINVAL;
+
+	r = ioctl(fd, BLKGETSIZE64, sectors);
+	(*sectors) /= 512;
+	close(fd);
+	return r;
+}
+
 //----------------------------------------------------------------