rgw: rest handlers for mdlog and datalog list dont loop

rgw: rest handlers for mdlog and datalog list dont loop

the do-while loops are unnecessary, because cls_log_list() will already
give us the requested number of entries, unless truncated. and these
rest operations return the truncated flag and last_marker, which the
client side is already using to run the same loop

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