[MIPS] Do not use drop_mmu_context to flusing other task's VIPT I-cache.

c-r4k.c and c-sb1.c use drop_mmu_context() to flush virtually tagged
I-caches, but this does not work for flushing other task's icache.  This
is for example triggered by copy_to_user_page() called from ptrace(2).
Use indexed flush for such cases.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>