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().

Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>