exofs: Support for RAID5 read-4-write interface.

The ore need suplied a r4w_get_page/r4w_put_page API
from Filesystem so it can get cache pages to read-into when
writing parial stripes.

Also I commented out and NULLed the .writepage (singular)
vector. Because it gives terrible write pattern to raid
and is apparently not needed. Even in OOM conditions the
system copes (even better) with out it.

TODO: How to specify to write_cache_pages() to start
      or include a certain page?

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>