sh: Rework sh4_flush_cache_page() for coherent kmap mapping.

This builds on top of the MIPS r4k code that does roughly the same thing.
This permits the use of kmap_coherent() for mapped pages with dirty
dcache lines and falls back on kmap_atomic() otherwise.

This also fixes up a problem with the alias check and defers to
shm_align_mask directly.

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