rgw: calc md5 and compare if user provided appropriate header

Fixes: #8436
Backport: firefly

This was broken in ddc2e1a8e39a5c6b9b224c3eebd1c0e762ca5782. The fix
resurrects and old check that was dropped.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>