mm/page_alloc: simplify and cleanup pcp locking

The pcp locking relies on pcp_spin_trylock() which has to be used together
with pcp_trylock_prepare()/pcp_trylock_finish() to work properly on !SMP
!RT configs.  This is tedious and error-prone.

We can remove pcp_spin_lock() and underlying pcpu_spin_lock() because we
don't use it.  Afterwards pcp_spin_unlock() is only used together with
pcp_spin_trylock().  Therefore we can add the UP_flags parameter to them
both and handle pcp_trylock_prepare()/finish() within.

Additionally for the configs where pcp_trylock_prepare()/finish() are
no-op (SMP || RT) make them pass &UP_flags to a no-op inline function. 
This ensures typechecking and makes the local variable "used" so we can
remove the __maybe_unused attributes.

In my compile testing, bloat-o-meter reported no change on SMP config, so
the compiler is capable of optimizing away the no-ops same as before, and
we have simplified the code using pcp_spin_trylock().

Link: https://lkml.kernel.org/r/20251015-b4-pcp-lock-cleanup-v2-1-740d999595d5@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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