Kconfig: switch CONFIG_SYSFS_SYCALL default to n

This odd system call will be removed in the future. Let's decouple it
from CONFIG_EXPERT and switch the default to n as a first step.

Link: https://lore.kernel.org/20250415-dezimieren-wertpapier-9fd18a211a41@brauner


Signed-off-by: Christian Brauner <brauner@kernel.org>