keys: use kmalloc_flex in user_preparse

keys: use kmalloc_flex in user_preparse

Use kmalloc_flex() when allocating a new struct user_key_payload in
user_preparse() to replace the open-coded size arithmetic and to keep
the size type-safe.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://lore.kernel.org/r/20260504093058.49720-3-thorsten.blum@linux.dev
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>