LU-9679 ptlrpc: list_for_each improvements.

LU-9679 ptlrpc: list_for_each improvements.

1/ use list_for_each_entry_safe() instead of
   list_for_each_safe() and similar.

2/ use list_first_entry() and list_last_entry() where appropriate.

3/ When removing everything from a list, use
   while ((x = list_first_entry_or_null()) {
   as it makes the intent clear

Linux-commit: ef8e5dbbb09035a0c41aa47a328e6248702d4d2b

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I490afcb70d18170e4ecc5d062d327d141668d3db
Reviewed-on: https://review.whamcloud.com/39133
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>