Fix concat operation doesn't honor dfs.permissions.enabled

In concat RPC, it will call FSDirConcatOp::verifySrcFiles() to check the source files. In this function, it would make permission check for srcs. Whether do the permission check should be decided by dfs.permissions.enabled configuration. And the 'pc' parameter is always not null.

So we should change 'if (pc != null)' to 'if (fsd.isPermissionEnabled())'.

// permission check for srcs
if (pc != null) {
  fsd.checkPathAccess(pc, iip, FsAction.READ); // read the file
  fsd.checkParentAccess(pc, iip, FsAction.WRITE); // for delete
}