kernel.h: Move upper_*_bits() and lower_*_bits() to wordpart.h

kernel.h: Move upper_*_bits() and lower_*_bits() to wordpart.h

The wordpart.h header is collecting APIs related to the handling
parts of the word (usually in byte granularity). The upper_*_bits()
and lower_*_bits() are good candidates to be moved to there.

This helps to clean up header dependency hell with regard to kernel.h
as the latter gathers completely unrelated stuff together and slows
down compilation (especially when it's included into other header).

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240214172752.3605073-1-andriy.shevchenko@linux.intel.com
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>