nodemask: Switch from inline to __always_inline

'inline' keyword is only a recommendation for compiler. If it decides to
not inline nodemask functions, the whole small_const_nbits() machinery
doesn't work.

This is how a standard GCC 11.3.0 does for my x86_64 build now. This patch
replaces 'inline' directive with unconditional '__always_inline' to make
sure that there's always a chance for compile-time optimization. It doesn't
change size of kernel image, according to bloat-o-meter.

[[ Brian: split out from:
      Subject: [PATCH 1/3] bitmap: switch from inline to __always_inline
      https://lore.kernel.org/all/20221027043810.350460-2-yury.norov@gmail.com/
   But rewritten, as there were too many conflicts. ]]
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>