libbpf: Replace AF_ALG with open coded SHA-256

libbpf: Replace AF_ALG with open coded SHA-256

Reimplement libbpf_sha256() using some basic SHA-256 C code.  This
eliminates the newly-added dependency on AF_ALG, which is a problematic
UAPI that is not supported by all kernels.

Make libbpf_sha256() return void, since it can no longer fail.  This
simplifies some callers.  Also drop the unnecessary 'sha_out_sz'
parameter.  Finally, also fix the typo in "compute_sha_udpate_offsets".

Fixes: c297fe3e9f99 ("libbpf: Implement SHA256 internal helper")
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://lore.kernel.org/r/20250928003833.138407-1-ebiggers@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>