xen/list: Remove prefetching

xen/list: Remove prefetching

Xen inherited its list infrastructure from Linux.  One area where has fallen
behind is that of prefetching, which as it turns out is a performance penalty
in most cases.

Prefetch of NULL on x86 is now widely measured to have glacial performance
properties, and will unconditionally hit on every hlist use due to the
termination condition.

Cross-port the following Linux patches:

  75d65a425c (2011) "hlist: remove software prefetching in hlist iterators"
  e66eed651f (2011) "list: remove prefetching from regular list iterators"
  c0d15cc7ee (2013) "linked-list: Remove __list_for_each"

to Xen, which results in the following net diffstat on x86:

  add/remove: 0/1 grow/shrink: 27/83 up/down: 576/-1648 (-1072)

(The code additions comes from a few now-inlined functions, and slightly
different basic block padding.)

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <julien@xen.org>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>