Move check for loop device inside crypt_loop_backing_file.

It also fixes minor regression where we return backing file
for partition on top of loop device when prompting for passphrase.
Partition on loop has different major number so it should not be
considered loop device at all.

lib/utils_loop.c

@@ -252,7 +252,12 @@ static char *_sysfs_backing_file(const char *loop)
 
 char *crypt_loop_backing_file(const char *loop)
 {
-	char *bf = _sysfs_backing_file(loop);
+	char *bf;
+
+	if (!crypt_loop_device(loop))
+		return NULL;
+
+	bf = _sysfs_backing_file(loop);
 	return bf ?: _ioctl_backing_file(loop);
 }