Patch to fix mkfs.btrfs on partitioned loop devices.

fix-partitioned-loop-device.patch

diff --git a/utils.c b/utils.c
index f10c178..04207b5 100644
--- a/utils.c
+++ b/utils.c
@@ -937,22 +937,20 @@ static int is_loop_device (const char* device) {
 static int resolve_loop_device(const char* loop_dev, char* loop_file,
 		int max_len)
 {
-	int ret;
-	FILE *f;
+	int fd;
 	char fmt[20];
 	char p[PATH_MAX];
 	char real_loop_dev[PATH_MAX];
+	struct loop_info64 lo64;
 
-	if (!realpath(loop_dev, real_loop_dev))
+	if (!(fd = open(loop_dev, O_RDONLY)))
 		return -errno;
-	snprintf(p, PATH_MAX, "/sys/block/%s/loop/backing_file", strrchr(real_loop_dev, '/'));
-	if (!(f = fopen(p, "r")))
+	if (ioctl(fd, LOOP_GET_STATUS64, &lo64) != 0)
 		return -errno;
 
-	snprintf(fmt, 20, "%%%i[^\n]", max_len-1);
-	ret = fscanf(f, fmt, loop_file);
-	fclose(f);
-	if (ret == EOF)
+	memcpy(loop_file, lo64.lo_file_name, strlen(lo64.lo_file_name) + 1);
+
+	if (close(fd) != 0)
 		return -errno;
 
 	return 0;