rgw: use get_current_index() instead of log_to_index_layout()

rgw: use get_current_index() instead of log_to_index_layout()

several places were getting the current index layout indirectly
with layout.logs.back() and rgw::log_to_index_layout(). use
get_current_index() instead so we don't rely on layout.logs, which may
be empty for indexless buckets

Signed-off-by: Casey Bodley <cbodley@redhat.com>