LU-6142 lustre: use list_first_entry() in lustre subdirectory.

LU-6142 lustre: use list_first_entry() in lustre subdirectory.

Convert
  list_entry(foo->next .....)
to
  list_first_entry(foo, ....)

in 'lustre'

In several cases the call is combined with
a list_empty() test and list_first_entry_or_null() is used

Signed-off-by: Mr. NeilBrown <neilb@suse.de>
Change-Id: I27b8b55cac2cfeaf95bb66930958c49ad422156e
Reviewed-on: https://review.whamcloud.com/44338
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>