arm64: futex.h: Add missing PAN toggling

futex.h's futex_atomic_cmpxchg_inatomic() does not use the
__futex_atomic_op() macro and needs its own PAN toggling. This was missed
when the feature was implemented.

Fixes: 338d4f49

 ("arm64: kernel: Add support for Privileged Access Never")
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>