lib/crypto: curve25519: Add at_least decoration to fixed-size array params

lib/crypto: curve25519: Add at_least decoration to fixed-size array params

Add the at_least (i.e. 'static') decoration to the fixed-size array
parameters of the curve25519 library functions.  This causes clang to
warn when a too-small array of known size is passed.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: "Jason A. Donenfeld" <Jason@zx2c4.com>
Link: https://lore.kernel.org/r/20251122194206.31822-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>