MIPS: c-r4k: Avoid dcache flush for sigtramps

Avoid the dcache and scache flush in local_r4k_flush_cache_sigtramp() if
the icache fills straight from the dcache.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/13802/


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