osd: explicit shard_id_t() and NO_SHARD

Whenever an integer or a shard_t needs to be converted in a shard_id_t,
replace the implicit conversion with an explicit call to the
constructor.

Whenever a ghobject_t::NO_SHARD and ghobject_t::no_shard() needs to be
converted into a shard_id_t, replace it with shard_id_t::NO_SHARD

Signed-off-by: Loic Dachary <loic@dachary.org>