rgw/rgw_dencoder: set identity_type when creating samples

rgw/rgw_dencoder: set identity_type when creating samples

otherwise identity_type is a random number on stack. and there is good
chance that it does not match with the one decoded from the encoded
blob.

this change should address the failures like:

**** rgw_log_entry test 2 binary reencode check failed ****
   ceph-dencoder type rgw_log_entry select_test 2 encode export /tmp/typ-O8hVYK2Gb
   ceph-dencoder type rgw_log_entry select_test 2 encode decode encode export /tmp/typ-G09itHHIC
   cmp /tmp/typ-O8hVYK2Gb /tmp/typ-G09itHHIC

The following tests FAILED:
	132 - check-generated.sh (Failed)
..

Fixes: https://tracker.ceph.com/issues/52278
Signed-off-by: Kefu Chai <kchai@redhat.com>