dm writecache: write at least 4k when committing

SSDs perform badly with sub-4k writes (because they perfrorm
read-modify-write internally), so make sure writecache writes at least
4k when committing.

Fixes: 991bd8d7

 ("dm writecache: commit just one block, not a full page")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>