During b9199e8b24
reivew it was suggested to use standard for loop
but when erasing element it causes increment on
invalid iterator, as role could have been erased
before.
This change brings back original code.
Fixes: https://github.com/scylladb/scylladb/issues/27422
Backport: no, offending commit not released yet
Closesscylladb/scylladb#27444