arm64: futex: Mask __user pointers prior to dereference

The arm64 futex code has some explicit dereferencing of user pointers
where performing atomic operations in response to a futex command. This
patch uses masking to limit any speculative futex operations to within
the user address space.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>