MIPS: Wire up sendmmsg and renumber setns syscall.

Renumbering was necessary because I had already wired up setns(2) in the
linux-mips.org tree in commit c3fce54644cabbb90700cc3acc040718a377f609
[MIPS: Wire up new sendmmsg syscall.] but the same syscall numbers were
used by 7b21fddd [ns: Wire up the setns
system call] resulting in a conflict.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>