We have to initiate CSumType from 1, which represents CSUM_NONE,
to be aligned with OSDMnitor's pool_opts_t handling.
So we have to explicitly check against CSUM_NONE to skip init_csum(),
which will set FLAG_CSUM and alloc memory for csum_data and thus
shall be avoided whenever it is possible.
Signed-off-by: xie xingguo <email@example.com>