MIPS' pipe(2) is weird, but MIPS' pipe2(2) is the same as everyone else's.

arch/mips/kernel/syscall.c has a special sysm_pipe wrapper, but there's
no special treatment of pipe2 because it carries no historical baggage.

Change-Id: I892c0f690b21992c8a48276a9b732126f18fc0ee