lib/crypto: blake2s: Rename blake2s_state to blake2s_ctx

lib/crypto: blake2s: Rename blake2s_state to blake2s_ctx

For consistency with the SHA-1, SHA-2, SHA-3 (in development), and MD5
library APIs, rename blake2s_state to blake2s_ctx.

As a refresher, the ctx name:

- Is a bit shorter.
- Avoids confusion with the compression function state, which is also
  often called the state (but is just part of the full context).
- Is consistent with OpenSSL.

Not a big deal, of course.  But consistency is nice.  With a BLAKE2b
library API about to be added, this is a convenient time to update this.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20251018043106.375964-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>