LU-12645 llite: Move readahead debug before exit

LU-12645 llite: Move readahead debug before exit

The core debug of ll_readahead() is before two return
conditions, which makes it really tricky to debug those
conditions.

Let's fix that.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ic3a3854527cad62c891c6a25029353a4742e555f
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51932
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>