folly/portability: ftruncate shouldn't change file offset

Summary:
From documentation in https://linux.die.net/man/2/ftruncate
> The file offset is not changed.

Reviewed By: Orvid, Skory

Differential Revision: D30843563

fbshipit-source-id: 9314576d927d9ddc07df6d0ceae394d148d7af94

folly/portability/Unistd.cpp

@@ -93,6 +93,10 @@ int fsync(int fd) {
 }
 
 int ftruncate(int fd, off_t len) {
+  off_t origLoc = _lseek(fd, 0, SEEK_CUR);
+  if (origLoc == -1) {
+    return -1;
+  }
   if (_lseek(fd, len, SEEK_SET) == -1) {
     return -1;
   }
@@ -104,6 +108,9 @@ int ftruncate(int fd, off_t len) {
   if (!SetEndOfFile(h)) {
     return -1;
   }
+  if (_lseek(fd, origLoc, SEEK_SET) == -1) {
+    return -1;
+  }
   return 0;
 }