nfs: Remove reference to generic_osync_inode from a comment

generic_file_direct_write() no longer calls generic_osync_inode() so remove the
comment.

CC: linux-nfs@vger.kernel.org
CC: Neil Brown <neilb@suse.de>
CC: "J. Bruce Fields" <bfields@fieldses.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>