Change the return types of bitops functions (ffs, fls, and fns) from
long to int. The expected return values are in the range [0, 64], for
which int is sufficient.

Additionally, int aligns well with the return types of the corresponding
__builtin_* functions, potentially reducing overall type conversions.

Many of the existing bitops functions already return an int and don't
need to be changed. The bitops functions in arch/ should be considered
separately.

Adjust some return variables to match the function return types.

With GCC 13 and defconfig, these changes reduced the size of a test
kernel image by 5,432 bytes on arm64 and by 248 bytes on riscv; there
were no changes in size on x86_64, powerpc, or m68k.
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>