arm64: uid16: fix __kernel_old_{gid,uid}_t definitions

Whilst native arm64 applications don't have the 16-bit UID/GID syscalls
wired up, compat tasks can still access them. The 16-bit wrappers for
these syscalls use __kernel_old_uid_t and __kernel_old_gid_t, which must
be 16-bit data types to maintain compatibility with the 16-bit UIDs used
by compat applications.

This patch defines 16-bit __kernel_old_{gid,uid}_t types for arm64
instead of using the 32-bit types provided by asm-generic.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>