sync: store the num_fences from first SYNC_IOC_FILE_INFO

Fixes a bug with the signal time of devices using the
modern sync file interface. The bug only affects kernels running
an Android kernel 4.9 and later.

b/63395253

Test: tests/sync_test.cpp

Change-Id: I6fb00bcb8e16a3268c357153edd8e35a44546caa

libsync/sync.c

@@ -217,6 +217,8 @@ static struct sync_file_info *modern_sync_file_info(int fd)
                   local_info.num_fences * sizeof(struct sync_fence_info));
     if (!info)
         return NULL;
+
+    info->num_fences = local_info.num_fences;
     info->sync_fence_info = (__u64)(uintptr_t)(info + 1);
 
     err = ioctl(fd, SYNC_IOC_FILE_INFO, info);