LU-12137 llite: use ->iterate_shared() for readdir

LU-12137 llite: use ->iterate_shared() for readdir

Use the ->iterate_shared() method for readdir in llite, which has
been available since kernel commit v4.6-rc3-29-g6192269.

Remove duplicate autoconf check for the "iterate_shared" method.

Fixes: e41bdca7559 ("LU-11071 build: Add server build support for Ubuntu 18.04")
Test-Parameters: clientdistro=sles12sp3
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I4dcec7c8ce1a5dbcc1e0ebc74ac47d1b7a4cab07
Reviewed-on: https://review.whamcloud.com/34556
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>