Use the 32-bit compat keyctl() syscall wrapper on Sparc64 for Sparc32 binary
compatibility.

Without this, keyctl(KEYCTL_INSTANTIATE_IOV) is liable to malfunction as it
uses an iovec array read from userspace - though the kernel should survive this
as it checks pointers and sizes anyway.

I think all the other keyctl() function should just work, provided (a) the top
32-bits of each 64-bit argument register are cleared prior to invoking the
syscall routine, and the 32-bit address space is right at the 0-end of the
64-bit address space.  Most of the arguments are 32-bit anyway, and so for
those clearing is not required.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: sparclinux@vger.kernel.org
cc: stable@vger.kernel.org