Explicitly cast to 64bit integer when calculating filesystem size

Else the result may overflow on platforms that have 32bit long.

Bug: 165925766
Test: Presubmit
Change-Id: I3018f0bd9846651848bd9b3645f2eeaa5b61c646
diff --git a/fs_mgr/fs_mgr_overlayfs.cpp b/fs_mgr/fs_mgr_overlayfs.cpp
index cdbadc9..1134f14 100644
--- a/fs_mgr/fs_mgr_overlayfs.cpp
+++ b/fs_mgr/fs_mgr_overlayfs.cpp
@@ -194,7 +194,7 @@
     static constexpr unsigned long kSizeThreshold = 8 * 1024 * 1024;  // 8MB
 
     return (vst.f_bfree >= (vst.f_blocks * kPercentThreshold / 100)) &&
-           (vst.f_bfree * vst.f_bsize) >= kSizeThreshold;
+           (static_cast<uint64_t>(vst.f_bfree) * vst.f_frsize) >= kSizeThreshold;
 }
 
 const auto kPhysicalDevice = "/dev/block/by-name/"s;