x86/uaccess: Add missing __force to casts in __access_ok() and valid_user_address()

Sparse complains about losing the __user address space due to the cast to
long:

  uaccess_64.h:88:24: sparse: warning: cast removes address space '__user' of expression

Annotate it with __force to tell sparse that this is intentional.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240304005104.677606054@linutronix.de