MIPS: ftrace: Fix icache flush range error

MIPS: ftrace: Fix icache flush range error

In 32-bit mode, the start address passed to flush_icache_range is
shifted by 4 bytes before the second safe_store_code() call.

This causes system crash from time to time because the first 4 bytes
might not be flushed properly. This bug exists since linux-3.8.

Also remove obsoleted comment while at it.

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: rostedt@goodmis.org
Cc: fweisbec@gmail.com
Cc: mingo@redhat.com
Cc: Qais.Yousef@imgtec.com
Patchwork: https://patchwork.linux-mips.org/patch/6586/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>