[POWERPC] spufs: Extract the file descriptor search logic in SPU coredump code

Extract the logic for searching through the file descriptors for spu contexts
into a separate routine, coredump_next_context(), so we can use it elsewhere
in future.  In the process we flatten the for loop, and move the NOSCHED test
into coredump_next_context().

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>