sh: Selective flush_cache_mm() flushing.

flush_cache_mm() wraps in to flush_cache_all(), which is rather
excessive given that the number of PTEs within the specified context
are generally quite low.  Optimize for walking the mm's VMA list and
selectively flushing the VMA ranges from the dcache. Invalidate the
icache only if a VMA sets VM_EXEC.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>