KEYS: fix overflow in keyctl_pkey_params_get_2()

KEYS: fix overflow in keyctl_pkey_params_get_2()

The length for the internal output buffer is calculated incorrectly, which
can result overflow when a too small buffer is provided.

Fix the bug by allocating internal output with the size of the maximum
length of the cryptographic primitive instead of caller provided size.

Link: https://lore.kernel.org/keyrings/20260531024914.3712130-1-jarkko@kernel.org/
Cc: stable@vger.kernel.org # v4.20+
Fixes: 00d60fd3b932 ("KEYS: Provide keyctls to drive the new key type ops for asymmetric keys [ver #2]")
Reported-by: Alessandro Groppo <ale.grpp@gmail.com>
Tested-by: Alessandro Groppo <ale.grpp@gmail.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>