arm64: compat: use compat_uptr_t type for compat_ucontext.uc_link

struct compat_ucontext * is a 64-bit pointer, so we need to use a
compat_uptr_t instead to avoid declaring a structure incompatible with
what AArch32 userspace expects.

Cc: <stable@vger.kernel.org>
Reported-by: Edmund Grimley-Evans <Edmund.Grimley-Evans@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>