cpumask: Switch from inline to __always_inline

On recent (v6.6+) builds with Clang (based on Clang 18.0.0) and certain
configurations [0], I'm finding that (lack of) inlining decisions may
lead to section mismatch warnings like the following:

  WARNING: modpost: vmlinux.o: section mismatch in reference:
  cpumask_andnot (section: .text) ->
  cpuhp_bringup_cpus_parallel.tmp_mask (section: .init.data) ERROR:
  modpost: Section mismatches detected.

or more confusingly:

  WARNING: modpost: vmlinux: section mismatch in reference:
  cpumask_andnot+0x5f (section: .text) -> efi_systab_phys (section:
  .init.data)

The first warning makes a little sense, because
cpuhp_bringup_cpus_parallel() (an __init function) calls
cpumask_andnot() on tmp_mask (an __initdata symbol). If the compiler
doesn't inline cpumask_andnot(), this may appear like a mismatch.

The second warning makes less sense, but might be because efi_systab_phys
and cpuhp_bringup_cpus_parallel.tmp_mask are laid out near each other,
and the latter isn't a proper C symbol definition.

In any case, it seems a reasonable solution to suggest more strongly to
the compiler that these cpumask macros *must* be inlined, as 'inline' is
just a recommendation.

This change has been previously proposed in the past as:

  Subject: [PATCH 1/3] bitmap: switch from inline to __always_inline
  https://lore.kernel.org/all/20221027043810.350460-2-yury.norov@gmail.com/

But the change has been split up, to separately justify the cpumask
changes (which drive my work) and the bitmap/const optimizations (that
Yury separately proposed for other reasons). This ends up as somewhere
between a "rebase" and "rewrite" -- I had to rewrite most of the patch.

According to bloat-o-meter, vmlinux decreases minimally in size (-0.00%
to -0.01%, depending on the version of GCC or Clang and .config in
question) with this series of changes:

gcc 13.2.0, x86_64_defconfig
-3005 bytes, Before=21944501, After=21941496, chg -0.01%

clang 16.0.6, x86_64_defconfig
-105 bytes, Before=22571692, After=22571587, chg -0.00%

gcc 9.5.0, x86_64_defconfig
-1771 bytes, Before=21557598, After=21555827, chg -0.01%

clang 18.0_pre516547 (ChromiumOS toolchain), x86_64_defconfig
-191 bytes, Before=22615339, After=22615148, chg -0.00%

clang 18.0_pre516547 (ChromiumOS toolchain), based on ChromiumOS config + gcov
-979 bytes, Before=76294783, After=76293804, chg -0.00%

[0] CONFIG_HOTPLUG_PARALLEL=y ('select'ed for x86 as of [1]) and
    CONFIG_GCOV_PROFILE_ALL.

[1] commit 0c7ffa32 ("x86/smpboot/64: Implement
    arch_cpuhp_init_parallel_bringup() and enable it")
Co-developed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Yury Norov <yury.norov@gmail.com>