Revert "MIPS: kernel: {ftrace,kgdb}: Set correct address limit for cache flushes"

This reverts commit 6ebda44f.

All icache flushes in this code paths are done via flush_icache_range(),
which only uses normal cache instruction. And this is the correct thing
for EVA mode, too. So no need to do set_fs(KERNEL_DS) here.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>