Abort migration early when not enough space.

Otherwise we potentially waste minutes of the users time copying
data that will never fit.

Also fix bug around storage calculation.  It's confusing, but f_bsize
is not the value you're looking for; the real block size is f_frsize.

Test: builds, boots
Bug: 27590986, 36840579
Change-Id: I77c63e259356824cc75a3adcf3f4af567efdc7aa

MoveTask.cpp

@@ -128,6 +128,12 @@ static status_t execCp(const std::string& fromPath, const std::string& toPath,
     uint64_t expectedBytes = GetTreeBytes(fromPath);
     uint64_t startFreeBytes = GetFreeBytes(toPath);
 
+    if (expectedBytes > startFreeBytes) {
+        LOG(ERROR) << "Data size " << expectedBytes << " is too large to fit in free space "
+                << startFreeBytes;
+        return -1;
+    }
+
     std::vector<std::string> cmd;
     cmd.push_back(kCpPath);
     cmd.push_back("-p"); /* preserve timestamps, ownership, and permissions */

Utils.cpp

@@ -432,7 +432,7 @@ status_t NormalizeHex(const std::string& in, std::string& out) {
 uint64_t GetFreeBytes(const std::string& path) {
     struct statvfs sb;
     if (statvfs(path.c_str(), &sb) == 0) {
-        return (uint64_t)sb.f_bfree * sb.f_bsize;
+        return (uint64_t) sb.f_bavail * sb.f_frsize;
     } else {
         return -1;
     }