copy_from_user might sleep so we can never call it when we have
a spinlock.

Moreover, it is not necessary in waiting for user interrupt, because
if multiple threads will call this function on the same interrupt,
each one will have it's own fence object inside the driver. The
user address might be the same, but it doesn't really matter to us,
as we only read from it.
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>