cifs: add FL_CLOSE to fl_flags mask in cifs_read_flock

FL_CLOSE is quite common when you close a file on which you hold a
lock. The spurious "Unknown lock flags" message in cFYI is
confusing in this case.

Reported-by: Alexander Bokovoy <abokovoy@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <sfrench@us.ibm.com>