os/bluestore: don't re-initialize csum-setting for existing blobs

os/bluestore: don't re-initialize csum-setting for existing blobs

The global checksum setting may change, e.g., from NONE to CRC32,
which can cause improper re-initialization of the csum-settings of
existing blobs(e.g., partial write/overwrite may turn out to shrink
'csum_data').

We could develop some complicated solutions but for now let's not
bother since the above scenario is rare.

Fixes: http://tracker.ceph.com/issues/21175
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>