mm/page_alloc: remove IRQ saving/restoring from pcp locking

mm/page_alloc: remove IRQ saving/restoring from pcp locking

Effectively revert commit 038a102535eb ("mm/page_alloc: prevent pcp
corruption with SMP=n").  The original problem is now avoided by
pcp_spin_trylock() always failing on CONFIG_SMP=n, so we do not need to
disable IRQs anymore.

It's not a complete revert, because keeping the pcp_spin_(un)lock()
wrappers is useful.  Rename them from _maybe_irqsave/restore to _nopin.
The difference from pcp_spin_trylock()/pcp_spin_unlock() is that the
_nopin variants don't perform pcpu_task_pin/unpin().

Link: https://lkml.kernel.org/r/20260227-b4-pcp-locking-cleanup-v1-2-f7e22e603447@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>