[POWERPC] spufs: Respect RLIMIT_CORE in spu coredump code

Currently the spu coredump code doesn't respect the ulimit, it should.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>