staging: lustre: llite: don't invoke direct_IO for the EOF case

The function generic_file_read_iter() does not check EOF
before invoke direct_IO callback. So we have to check it
ourselves.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8969
Reviewed-on: https://review.whamcloud.com/24552


Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>