Make KeyValueStore not use expected_write_size

expected_write_size in set_alloc_hint isn't the mean of name, now it's just the
size of object. It's not suitable for KeyValueStore to set strip size.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Merge pull request #1967 from yuyuyu101/kvstore-fix-set-alloc-hint

Make KeyValueStore not use expected_write_size

Reviewed-by: Sage Weil <sage@redhat.com>