[scan] ensure metadata file copy is same size

(re)writing metadata using ffmpeg may result in output files being a
smaller size IF the original metadata was null/zero padded - we
truncate the dst (in place copy) to match size

src/library/filescanner_ffmpeg.c

@@ -818,8 +818,12 @@ fast_copy(int fd_dst, int fd_src)
 #else
   // sendfile will work with non-socket output (i.e. regular file) on Linux 2.6.33+
   struct stat fileinfo = { 0 };
+  int ret;
   fstat(fd_src, &fileinfo);
-  return sendfile(fd_dst, fd_src, NULL, fileinfo.st_size);
+  ret = sendfile(fd_dst, fd_src, NULL, fileinfo.st_size);
+  if (ret < 0)
+    return -1;
+  return ftruncate(fd_dst, fileinfo.st_size);
 #endif
 }