crimson/os: use structured binding in loop

crimson/os: use structured binding in loop

also avoid using `map[key] = val` for setting an item in map, as, if
he key does not exist in map, `map[key]` would have to create a value
using its default ctor, and then call the `operator=(bufferlist&&)` to
set it.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>