crimson/osd: erase an element by iterator instead

crimson/osd: erase an element by iterator instead

we should not remove an element while iterating it in a map, as erasing
the element invalidates the iterator, which causes segmfault when we are
advancing it after erasing the dereferenced element.

in this change, an iterator is used for walking through the map, in
comparision with creating a to-be-removed list, this one is more
efficient and more idiomatic.

Signed-off-by: Kefu Chai <kchai@redhat.com>