selftests/mm: ensure non-global pkey symbols are marked static

The pkey tests define a whole lot of functions and some global variables. 
A few are truly global (declared in pkey-helpers.h), but the majority are
file-scoped.  Make sure those are labelled static.

Some of the pkey_{access,write}_{allow,deny} helpers are not called, or
only called when building for some architectures.  Mark them
__maybe_unused to suppress compiler warnings.

Link: https://lkml.kernel.org/r/20241209095019.1732120-11-kevin.brodsky@arm.com


Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Keith Lucas <keith.lucas@oracle.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>