MIPS: secure_computing, syscall audit: syscall number should in r2, not r0.

As it is, audit_syscall_entry() and secure_computing() get the
bogus value (0, in fact)

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-kernel@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1697/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>