selftests/mm: Fix unused function warning for aarch64_write_signal_pkey()

Since commit 49f59573 ("selftests/mm: Enable pkey_sighandler_tests
on arm64"), pkey_sighandler_tests.c (which includes pkey-arm64.h via
pkey-helpers.h) ends up compiled for arm64. Since it doesn't use
aarch64_write_signal_pkey(), the compiler warns:

In file included from pkey-helpers.h:106,
                 from pkey_sighandler_tests.c:31:
pkey-arm64.h:130:13: warning: ‘aarch64_write_signal_pkey’ defined but not used [-Wunused-function]
  130 | static void aarch64_write_signal_pkey(ucontext_t *uctxt, u64 pkey)
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~

Make the aarch64_write_signal_pkey() a 'static inline void' function to
avoid the compiler warning.

Fixes: f5b5ea51 ("selftests: mm: make protection_keys test work on arm64")
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Reviewed-by: Kevin Brodsky <kevin.brodsky@arm.com>
Link: https://lore.kernel.org/r/20241108110549.1185923-1-catalin.marinas@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>