sh: Fix argument page dcache flushing regression.

In the do_execve() path, argument page handling used to explicitly call
flush_dcache_page() for each page, this has since been reworked and
uses flush_kernel_dcache_page() instead, which is presently a nop.

Doing a simple modprobe/rmmod in a loop under busybox consistently
manages to crash without providing a sane flush_kernel_dcache_page()
implementation, so, plug in a simple implementation.

Signed-off-by: Carmelo Amoroso <carmelo73@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>