libsparse: remove static variables

Removes static variables in backed_block.c to allow multiple
sparse files to be open at the same time.

Change-Id: I012d8a424c6e21a7352408416adb7c72ee8add21
diff --git a/libsparse/sparse_file.h b/libsparse/sparse_file.h
index 05a78d9..fae1c16 100644
--- a/libsparse/sparse_file.h
+++ b/libsparse/sparse_file.h
@@ -23,6 +23,7 @@
 	unsigned int block_size;
 	int64_t len;
 
+	struct backed_block_list *backed_block_list;
 	struct output_file *out;
 };