xen: avoid type warning in xchg_xen_ulong

The improved type-checking version of container_of() triggers a warning for
xchg_xen_ulong, pointing out that 'xen_ulong_t' is unsigned, but atomic64_t
contains a signed value:

drivers/xen/events/events_2l.c: In function 'evtchn_2l_handle_events':
drivers/xen/events/events_2l.c:187:1020: error: call to '__compiletime_assert_187' declared with attribute error: pointer type mismatch in container_of()

This adds a cast to work around the warning.

Cc: Ian Abbott <abbotti@mev.co.uk>
Fixes: 85323a991d40 ("xen: arm: mandate EABI and use generic atomic operations.")
Fixes: daa2ac80834d ("kernel.h: handle pointers to arrays better in container_of()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Ian Abbott <abbotti@mev.co.uk>

arch/arm/include/asm/xen/events.h

@@ -16,7 +16,7 @@ static inline int xen_irqs_disabled(struct pt_regs *regs)
 	return raw_irqs_disabled_flags(regs->ARM_cpsr);
 }
 
-#define xchg_xen_ulong(ptr, val) atomic64_xchg(container_of((ptr),	\
+#define xchg_xen_ulong(ptr, val) atomic64_xchg(container_of((long long*)(ptr),\
 							    atomic64_t,	\
 							    counter), (val))