Fix intermittent upload failures on macOS

There were multiple problems with the FdManager::GetFreeDiskSpace() function
on macOS:
1) When calling statvfs(), f_frsize should be used instead of f_bsize when
converting available blocks to bytes.  This was causing the free space
calculation to be incorrect.
2) On macOS, fsblkcnt_t is a 32-bit integer.  Thus, when calculating available
disk space, there were frequently overflows.  This caused s3fs to incorrectly
determine that the cache location was out of space in the middle of a transfer
which caused uploads to fail.  Changing this to a uint64_t resolves the
problem.

src/fdcache.cpp

@@ -1908,7 +1908,7 @@ size_t FdManager::SetEnsureFreeDiskSpace(size_t size)
   return old;
 }
 
-fsblkcnt_t FdManager::GetFreeDiskSpace(const char* path)
+uint64_t FdManager::GetFreeDiskSpace(const char* path)
 {
   struct statvfs vfsbuf;
   string         ctoppath;
@@ -1930,12 +1930,12 @@ fsblkcnt_t FdManager::GetFreeDiskSpace(const char* path)
     S3FS_PRN_ERR("could not get vfs stat by errno(%d)", errno);
     return 0;
   }
-  return (vfsbuf.f_bavail * vfsbuf.f_bsize);
+  return (vfsbuf.f_bavail * vfsbuf.f_frsize);
 }
 
 bool FdManager::IsSafeDiskSpace(const char* path, size_t size)
 {
-  fsblkcnt_t fsize = FdManager::GetFreeDiskSpace(path);
+  uint64_t fsize = FdManager::GetFreeDiskSpace(path);
   return ((size + FdManager::GetEnsureFreeDiskSpace()) <= fsize);
 }
 

src/fdcache.h

@@ -194,7 +194,7 @@ class FdManager
     fdent_map_t            fent;
 
   private:
-    static fsblkcnt_t GetFreeDiskSpace(const char* path);
+    static uint64_t GetFreeDiskSpace(const char* path);
     void CleanupCacheDirInternal(const std::string &path = "");
 
   public: