lib/list: Fix handling of empty lists

The current list_for_each_entry_safe marco SEGVs on empty lists; the
list_entry will give us a NULL tmp on the first iteration.

This change removes the use of list_entry, as we're effectively
by-passing its NULL return semantics with our own.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Geoff Levand <geoff@infradead.org>