sha256_generic: make internal functions a little more private

Linux 6.18 has conflicting prototypes for various sha256_* and sha512_*
functions, which we get through a very long include chain. That's tough
to fix right now; easier is just to rename our internal functions.

Sponsored-by: https://despairlabs.com/sponsor/
Signed-off-by: Rob Norris <robn@despairlabs.com>